Ive got the 18hp right now and I feel it is a bit small. I dont have many issues with clogging or bogging down with dry leaves but I sometimes have trouble with really wet leaves. Also sucking up grass can be a bit slow. It doesn't clog, it just takes it in slow. If you plan on feeding it with a rake then the 18hp will be ok but if you use the hose to suck right out of the pile or you mow everything first, the 25hp would be better. The more hp and larger hose will also help with wet leaves.
I will say that I am happy with my billy goat loader though. It does a great job at mulching everything so I can really fit alot in the trailer. I still have to climb into the trailer and jump on the pile to pack everything down once or twice but loader does a good job of packing, especially when they are wet. There is a guy around here who has a setup like yours but with the 25hp goat. His seems to work very well.