Will-
I notice that you did not mention the top forms module (based on sales) offered on SnowCovered:  XMOD.  We have used XMOD extensively for event registrations, payments, information inquires, surveys, as a front-end for custom DB searches and more.  It's been around forever and is a very robust forms module.

Many people prefer Dynamic forms to XMOD due to some XMOD limitations.  For me, XMOD's biggest drawback is the inability to do anything but perform SELECT queries with non-XMOD tables (i.e., you cannot use native XMOD to perform CREATE, UPDATE or DELETE queries for data outside of the actual XMOD form-submitted data).  XMOD does support extensions, though, so you can custom-code addon features if desired.

-mamlin

Will-
I'll take your reply to mean that you want to store data in external XML files, not just as XML (XMOD natively stores data as XML in the DB). 

If so, you can probably rule out H2O as it's original purpose was specifically to store data in the DB as dynamically-created tables as an alternative to XMOD's XML data format.  The tables return much better SQL performance compared to parsing XML but if you create a lot of custom forms then you're going to end up with a VERY cluttered DB under H2O.  Some of the other forms modules operate via dynamically-created tables as well.

Be aware than XMOD imports and exports XML but only via admin action, not automatically.  If your "external XML file" requirement is due to a need to import existing data, XMOD can do it.  There may be some possibilities with an addon like ProcLabel but I'm not aware of a method to direct XMOD to use external XML files in place of the DB.

You might also be interested in this thread:
http://www.dotnetnuke.com/Community/Forums/tabid/795/forumid/118/threadid/209855/scope/posts/Default.aspx

Cheers!
-mamlin