Pricing (weighted 12%)
Gravity Forms
Personal License: $39/year
Use unlimited forms with unlimited entries for only one site. Features like Auto-responders, Export Entries, Spam Protection are available but there are no add-ons included.
Business License: $99/year
At this price point basic add-ons can be added and you can use the license on up to 3 websites.
Developer License: $199/year
You get everything with this license, including priority ticket support and access to advanced add-ons.
For all plans you will get a full year of support and updates from the date of purchase. If you don't renew your subscription Gravity will still work but you won't get any updates or access to support.
Gravity Forms is also a WordPress form plugin so if your website is built on another platform you might want to look at the other form builders.
There is no free trial, but a demo is available if you wish to get a feel of the Gravity Forms administration before you buy.
Formstack
Silver: $39/month
The starter plan lots of features like a drag and drop form builder, conditional logic, CSS and themes, 5 users, mobile apps, save & resume, security and email support and much more. All of this if you need to create under 20 forms with maximum 1000 submissions per form every month.
Gold: $99/month
With this plan you'll get a few more integrations (Hubspot, Google Analitycs and Salesforce), 10 users and you can build up to 100 forms with 10,000 for each form every month.
Platinum $249/month
This plan includes more advanced features like AB testing but you are limited in number of forms and submissions. Nevertheless it bumps up the number of forms to 1000, the monthly submissions per forms to 100.000 and the number of logins per account to 25.
There is no free plan but they have a 14 day free trial. You can also request a 20 minute demo that includes tutorials and tips to help you get started.
The features are pretty attractive but the maximum submissions per month might be a problem.
They offer 2 months free when you pay annually.
Design (weighted 12%)
Gravity Forms
Gravity Forms—like a number of other WordPress form plugins—lets you make forms inside of your WordPress Admin pages and it comes styled to work out of the box with just about any theme.
If you want to tweak the way your form looks you can use the Gravity Forms CSS classes under the advanced field settings to do so. Nevertheless this will require some coding skills so if you don't want to get your hands dirty try the demo forms first and see if they match with your WP theme.
Formstack
All forms are completely responsive and there are a enough templates available to get you started. The designs have a modern look and will easily integrate into your website design but if you wish to customize your themes HTML and CSS skills will be needed.
Features (weighted 12%)
Gravity Forms
With Gravity you can make multi-page forms, schedule forms and restrict number of entries, create WordPress posts from entries, save forms and finish filling them out later.
Lots of add-ons are available if you need to integrate payment platforms and other web apps with the builder.
Conditional logic is is a pretty powerful feature, it allows you to show and hide form fields/questions based on how users respond to previous questions.
Gravity Forms alos allows you to create WordPress posts from user entries or quickly build surveys and reader polls with open-ended text fields, drop down menus, checkboxes, numerical fields, and multiple-choice questions.
Formstack
Starting with the Silver plan you'll get access to all the basic features, like the WYSIWYG Drag and Drop editor, CSS and Themes, Conditional Logic, Multiple Users, Mobile, Apps, Save & Resume, Basic Integrations, Electronic Signatures, Form Analytics, Field Validation and Payment Processors.
Formstack is available for any platform so if you're not running a WordPress site this might be the choice. Another big plus is that you are only limited by the number of forms and submissions, so the forms you create can be used on more that one website.
Backend Features (weighted 12%)
Gravity Forms
Gravity Forms is a WordPress plugin, so your website has to run on WordPress in order to use it.
After you install the plugin you will find a new Forms section in the the WP admin menu where you'll be able to add new forms, edit your forms, and see their results.
A drag and drop editor is available and you can easily create, edit and share your forms.
Formstack
Formstack has a clean and uncluttered administration interface with a fresh and modern design.
In the backed you'll be able to access your Dashboard which contains reports like unique forms visits and conversion rates. In the Forms section you can organize your existing forms or create new ones. The next section is
Learn Formstack - an overview of every single walkthrough available to you, organized by category and difficulty level.
Extensibility (weighted 12%)
Gravity Forms
Gravity Forms has over 100 extensions and add-ons, giving you tons of third party options. The business licence gives you access to integrations (basic add-ons) with all the major email marketing services like (Campaign Monitor, MailChimp, AWeber, etc.). With the Developer licence you'll have access to the advanced add-ons.
The downside is that the add-ons are only available with the Business or Developer Licence so if you are running only one website go with Ninja Forms.
Formstack
Formstack has many web app integrations available for payments, data collection, email lists, advanced analytics insights all arranged in categories.
Dropbox, Aweber, MailChimp, Facebook, Google Analytics, PayPal are some of the most popular.
Flexibility (weighted 12%)
Gravity Forms
If you want to expand the uses for the plugin with add-ons you’ll need at least a business license at $99 or a developer license (for access to all add-ons) at $199.
You can get a full refund within the first 30 days of your purchase.
WordPress developers can customize Gravity Forms functionality using the Gravity Forms API.
Formstack
You can export all of your data or manually choose what data to export in CSV, RTF (Word), Excel, or PDF format.
With the Formstack iOS and Android app, you can take and process orders, collect or see sales contacts, and take survey information.
Statistics (weighted 12%)
Gravity Forms
Gravity Forms doesn't currently have reporting capabilities or other add-ons for use as an advanced survey.
Formstack
For every form some basic analytics data is avalaible: Unique Views, Conversions, Conversion Rate, Abandonments, Abandonment Rate. For more advanced analytics data the Google Analytics integration is available.
Support (weighted 12%)
Gravity Forms
Gravity forms has good documentation. If you encounter problems there are customer service forums on the support page and a FAQ section that you can check before submitting a support ticket. You can also reach them on Facebook and Twitter, for quick support questions, they usually respond within a few hours.
Formstack
Formstack offer support aver Email and Live Chat. DIYers are able to access the Support Center with a decent knowledgebase (with walkthroughs and video tutorials available), community help Forum (not very active) and a Blog (very active) as well are available.
All these support features are even available the Free level.