Thank you for your post.
I was hoping others would answer this as this is very subjective. That is, some users love the portability and small screen size of the iPhone, while others like the large screen size of the iPad. Since FileMaker Go is a free download, I would try it on one of your iOS devices and play with it a bit to get a feel. Also, I would recommend reading the FileMaker Go Development Guide at:
and the FileMaker Go Technical Brief at:
This may help you with your decision.