Shutterfly is defnitely better, but I might be biased because I work there Honestly though, here are the advantages:
1) Your guests do not need to create an account to view your pictures (which makes it easier for guests).
2) Shutterfly has share sites specifically for weddings and you can store videos on share sites too (snapfish might have this too-- not sure)
3) Shutterfly is the leader in photo books---and trust me you will want to make photo books with all the pictures from your share site (that you took and guests took). They are a great gift for mom/dad and bridal party! Shutterfly has better quality books, more design options and are easier to make on Shutterfly.
4) Shutterfly offers 100% satisfaction guarantee-- this sounds like an total advertisement--but this is so important. If you aren't happy with the way any of your products turn out --you can get your money back or order somethign else, etc.
Okay- that is my $.02.
Happy picture taking!