Despite what some snobs say 6x4.5 is a big jump up in negative size from 35mm. In my experience 6x4.5 is a great place to start. It is the lightest of the commonly available MF SLR equipment. Also unless you print square you are essentially shooting 6x4.5ish with a 6x6 camera. So to get a rectangular size advantage over 6x4.5 you have to jump up to 6x7... and that is a whole other world filled with rangerfinders and SLRS that are better kept firmly on a tripod.
The Bronica ETRSi is a good entry point because you can get it for a reasonable price and if you decide you want to upgrade you can sell it easily enough without taking too much of a hit.
Thanks Noble, I think I will go for a ETRSi, I found a couple which hopefully will stay at lower prices!