It's all about light gathering. I guess all things equal in terms of focal length and eyepiece used you'd see more in the 8". The advice I had was buy as big as you can. I'm glad I did as portability is not a concern for me.

I bought an 8" - its great but a 10" would have been better and then of course a 12" would have been better still need I say more :) whatever you buy you will always wish you had bought bigger.

Cheers Paul