My agenda is to learn to handwrite like a native. Perhaps I won't know as many characters, but I would like to be able to render the characters I do know in a natural, native-like cursive.
To that end, a pen Xing would be helpful, since it is the closest thing to a standard for modern cursive handwriting (if I understand correctly). Plus, in Xing, the stroke-order is often evident from the finished character without animation. However, an animated Pen Xing would really take the cake.
Kai is certainly better than the built-in for learning handwriting, but less useful than Xing could be imo.
Cao may be interesting for historical or artistic reasons, but I don't see it being all that useful for the modern learner. Perhaps it has a place somewhere in the 字 tab.
What is missing from the poll is a modern cursive script; i.e., the way (or one of the ways) that modern Chinese write quickly using a pen. Something like
http://db.tt/BjKXuZWA. (Correct me if I'm wrong, but this handwriting looks more 'running' than most Xingshu examples I've seen online.)
-Chris