I shoot mostly 6x7 (RZ67) but the drawback is that it's all but impossible (well, it's very expensive, i.e. $1k to $5k) to buy a projector+lens in that size. For B&W and C41, it's awesome. If you shoot and project chromes, 6x6 might be a better option since 6x6 projectors are readily available for ~$200.
Don't get something that requires 220 film, that market is over. There's 120 now and that's it.
My opinionated summary of the 6x7 options:
- RZ67: easy to do portrait+landscape, good lenses & availability, flash sync at all speeds (leaf shutter). Very bulky; prism is heavy. Interchangeable backs allow infinite film-changes in the field - I typically carry colour, B&W and IR all loaded.
- RB67: like RZ but notably heavier, shutters are mechanically timed so can be less accurate, no A-mode metering, some better lenses not available.
- Pentax: heavy, bouncy. You MUST get the MLU version and even then, the focal-plane shutter will soften your images at some speeds depending on your tripod damping. Requires L-plate to shoot portrait. Very good/fast lens selection.
- Mamiya 7: probably the ultimate portable landscape machine, especially for the 43mm lens. Stupidly expensive, limited lens selection, not much good for closeups.
- Bronicas: I dunno.
If I were you, I would definitely get something with leaf shutters. If you ever shoot portraits in sunshine you will appreciate the ability to sync at reasonable speeds (1/400 being max speed on RZ vs 1/30 being x-sync on Pentax). Leaf shutters don't vibrate so you can shoot MLU on the shittiest flimsiest tripod and get perfectly sharp results if you wait 15s for the mirror slap to damp out.
Definitely get yourself setup to develop film at home, even if only B&W for a start. Colour is eminently doable too with a little extra equipment.
By all means look at 4x5 in addition - I have a Toyo 45A for that purpose. It's much more powerful and flexible but the slowness means you will occasionally miss shots that you would have trivially got with a MF SLR when the light is changing fast. You know, that two minutes between when the sun appears under the edge of the overcast and then disappears behind the hill, when the light is best. You'll get one composition with the 4x5, but if you're careful and quick, you might get 3 or 4 notably different compositions from a MF system. 4x5 is also a huge amount more work to process; just loading and unloading film holders is a huge time-sink.
If you like shooting in colour, LF will make your wallet hurt. Think $4+ vs 80c per shot; worse if you're paying someone else to develop. The price differential is nowhere near as bad in B&W.
If you're willing to pay an extra $500 upfront, you can put a 6x12 back on a 4x5 camera and shoot (colour) rollfilm in it cheaply, getting most of the benefits of LF without the hassle of paying for and dealing with sheet film.