I really like the way the 550 tank looks with it. To me it loks like the tank flows with the lines of the seat and fenders better. But, how much of a fuel difference are we talking here? Do you gain a lot more with the 750 tank? Anyway, I'd go with the 550 tank myself, but it's your bike bro. Build it how you want it, not what someone tells you it should look like.