My application is portraits and I just want to use a normal lens. I like the look of shallow DOF (but not too shallow) combined with a slight tilt. It is hard for me to judge which format is best for my application.

4x5 would be the cheapest way to go but most normal lenses are f5.6 and I believe there are some f4.5's out there...the faster lenses are generally more rare with the exception of the Aero Ektar. This format might be suiteable but it is hard to judge because I base my decisions from flickr and most people, in my opinion, tilt too much if they were to use it.

8x10 would be the most expensive way to go and I know that it is capable in providing the "look" that I want but the cameras are larger and the running cost is great.

5x7 seems like a good compromise and as a bonus I can make contact prints (well I can with 8x10 but not with 4x5 for my liking). From flickr I see the look that I want more often with 5x7 than I do with 4x5.

Lenses
4x5: 150mm-180mm
5x7: 210mm-240mm
8x10: 300mm-360mm

I generally prefer wider and I was thinking of mostly Tessars and Plasmats. I'm not too interested in the Aero Ektar because it seems too soft and too low in contrast wide-open but that lens is a seperate discussion. I would also prefer 150mm over 180mm in this format.