PDF Forms are not adaptive and neither are HTML5. Adaptive Forms are. There are differences between PDF and HTML5 renderings not from a viewing perspective but from an approach perspective for example to work offline with html5 the AEM forms app would be needed and printing needs a different approach to nam just two. Yes web services can be called from html5 forms. HTML5 can be converted to PDF and a form created out of that result. An XDP form can be rendered as PDF or html5 and used as the form model for an Adaptive Form but it's recommended to use a schema.
When we talk about adaptive forms, there phrase "adaptive" means the forms are designed to react dynamically to user behavior, for example by adding or removing fields during the entry process.
The answer to effort for is for HTML Forms the effort is a little less as compared to AF because xdp can be rendered as HTML form Answer to points 2 and 3 is Yes
With AEM Forms 6.2 you have a couple of forms development choices: a) Create a fillable XDP form using Designer that can be rendered as both HTML and PDF; b) Create an Adaptive Form using the Adaptive Forms editor within AEM.
Now, if you're creating a new form and it's primary destination is the web and not offline or print then use Adaptive Forms. If you have existing PDF forms that you want to expose to the web quickly, then render your XDP's as HTML. That said, the experience with Adaptive Forms is much - much better from a user point of view and from a developer perspective, much more flexible.
From the web services perspective, yes, both PDF and HTML can call web services nicely. The architecture is a little different though. PDF web service calls are called directly from the client (Reader) but web service calls from HTML rendered from PDF is actually proxied to the server. Web services from Adaptive Forms are a snap because you have 100% control over the HTML that is rendered and the Client Libraries feature of AEM let's you create the JS that will make the AJAX calls for you (use JSON as all your return data to make your life easier).
Note:- We are clearing the unanswered post (sorry for late reply here).