Ive got a 25, and the boat size you mention is about like mine. Definitely go with the 25, then you can reasonably fish places like the Delta or Clear Lake, or whereever really. You will not be able to do that with a 15. I built a cheap motor stand that sits behind my boat in the garage. It takes me 5 minutes to get it on and off. Its not that heaavy.

However, is you get a 25 four stroke than you may start getting into too much weight.