I cannot emphasize this enough, definitely shoot with each before you make a decision.
The thing I like the most about shooting with medium or large format is that it makes you slow down and take your time with each shot. One of the reasons I have pretty much abandoned 35mm is that when I shoot that format, I end up with hundreds of shots to sort through and only a few that are worth keeping. With large format, I'm happy with about 50% of the photos I take.
I don't have enough experience with medium format to add onto what has already been said, I think that's been summed up pretty well until you have more specific questions. I do have two friends who shoot with the Pentax 67 (one of them exclusively), and that camera produces beautiful images.
As far as large format goes...I wouldn't know where to start without specific questions from you. Feel free to ask anything and I'll do my best to answer! One hurdle you will want to consider is processing, though. 4x5 is pretty expensive to send out (about $3 a negative), and my scans are usually 300-600MB from each negative--although that's at max resolution. For developing yourself, you have a few options, and I'm happy to talk about them with you if you're curious.
The reason I shoot large format is because you can do things with it that are impossible in any other format. With my monorail camera I can independently control perspective and focal plane, and do pretty much whatever I want with either. There's the obvious image quality advantage too, but with a good scan that doesn't become evident until you get into large prints (24"+). There's a lot that each format offers, and you'll make some definite sacrifices going with either, along with putting at least a few hundred dollars into getting set up.
So yeah, ask me whatever and I'll try to give you answers! I'll put up a few examples here too, just to try to show how I use each.
Hasselblad 503
Toyo 45 GII