imo it depends on what your looking for really....what kind of bass do you want? will this be a streat beater or comp only? will it be a wall vehicle or not? in the past it seems 10,s tend to punch harder and the 15,s will drop lower so if you like punchy bass prolly the 10,s if you like your shit to drop low then prolly the 15,s but alot of that depends on the box you build also you can tune your shit to go either way but the 10,s will still be a lil punchy and the 15,s will still hit a lil low....all depends on what your going for....imo you will be able to get louder with the 4 10.s but they will prolly need close to 5000 rms to reach thier potential....and also imo the 2 15,s will prolly sound a little better with the 3500 but i do like the way 15,s sound better than i do 10,s so maybe im a lil biased anyway hope this helps you