25hp 2-Stroke vs 20hp 4-Stroke for 4m RIB?
20hp & 30hp fourstroke Tohatsus are very different engines.
20hp is a twin cylinder carbed, and the same as the 15hp, still classed as a portable engine. I would doubt very much you'd pull a skier with that. Certainly won't be as punchy 25 two stroke.
The 30hp, is a 3 cylinder efi, a proper engine, and should more than match your 25 2 stroke. For what you are after I wouldn't even consider the 20hp. If your using a trailer, i can't see an extra 20kg making much difference to launching/recovery.