They believe it's a bit too heavy and big to be fitted with a .15 engine.
Darock, with your experience, can you give me a honest comments on whether I should stick to a .15 or upgrade to a .25?
If they're experienced flyers they're probably right. If they've also got experience with a similar swap at that size, they're certainly right.
I've used a .20s on a .15 sized airplane and it worked, but I built the airplane from plans and reinforced it as I built. The airplane was harder to fly than before, but it wasn't a trainer nor was it large for a .15 sized airplane.
An Edge is an aerobatic airplane so you're not expecting it to be slow and stable. Go for it.
I gave you honest comments on what to expect generally. And I'm confident those things will happen. But with my experience I know for a fact that nobody can predict the weather more than a week away nor what exactly will happen if you change a model airplane.
About the only thing I'd add to my previous generalizations would be that you look for a .20 that's as close to the weight of your .15 as you can find. And the same advice if you decide on a .25.