Couldn't agree more with what was already said. I bought a 503CW recently at first with one 120 back. I now have three of them. There is just more variety of 120 film out there and that is why originally chose the 120 back.
Now that I have more or less standardized on my film, and they are available in 220, I have been thinking of getting a couple 220 backs for my 'standard' film and using the 120 backs for the 'neat' stuff.