Hi Jimbo, Find a Kayak dealer in your area and try some out. Yes its fairly easy to flycast out of kayaks. After using both I give the nod to sit on tops for that (but, not all things).
You did not mention your size, and that is really important.
The "fat/skinny" yak statement is not really determined by length at all. In gereral a wider kayak will be more enjoyable once you get to your area for fishing. It will be a better more comfortable platform.
The less wide kayak will be more enjoyable getting you there and back.
Longer is faster, smoother & glides. Shorter is quicker,turns better.
I can suggest a list of sit in or on, flyfishing friendly kayaks but need more info. on your size,storage,transporting, etc