One little thing has been bugging me and that is you don't appear to have a 35mm slide projector atm, so maybe you haven't seen how amazing projected 35mm slides are?
A 35mm slide projected onto an old fashioned screen about 5ft wide is awsome. To get the same awsomeness from 120mm you will need a screen the size of a bus.
I went to 120 not because I wanted to project (which is still a great way to show photos) but because I wanted sharper scans and prints. So if you want to project, use 35mm, or, if you want super sharpness in printing or scanning, use 120?