I have always had a soft spot for the old Pentax 6x7 (the one like a giant SLR). But as an old pro I have learned that these days (with CAD CAM and all) there is virtually nothing to separate most cameras in terms of quality and it comes down to personal preference. You only notice performance differences when working at the technical limits of your equipment and you don't often do that - even at top end pro.
The best advice (indeed the ONLY advice) I can give is: you must have a camera you feel comfortable with. You can't begin to take pictures worth a damn if all the time you have your mind on the bit of technology in your hands.
You wouldn't believe the number of amateurs that have come up to me over the years, raved on at length about the fabulously expensive camera they just bought and how it can do everything including making them breakfast in bed. Then they go thoughtful and plaintively ask why, given this wonderful piece of kit, there pictures are not really any better and they still haven't figured out what THIS button does. If I like them I take them aside and reveal the secret of the Universe. They usually burst into tears; "you mean I've wasted my money!?!" No but you are going to have to put in a whole lot more work.
Don't bother reading the reviews in the magazines - those guys are PAID to say those things (often by the manufacturers).
Don't buy a particular camera because everyone you know has that model. What's right for them does not mean it's right for you.
Pick up and play with every camera you can. Sooner or later you will pick up one that feels like it belongs in your hands. Buy THAT one (whatever the price).
My cameras - especially my 35mm's (Olympus OM's) - feel like part of me. I can operate them with my eyes shut, half asleep and blind drunk. I don't have to look because I can feel it's working right - and you can feel when something changes and then you look. I know exactly what they are going to give me so I can concentrate totally on what is in front of the lens. THIS is the main difference between am and pro.
Same with format. If you don't like the proportions don't buy it. Get one that you think looks right. I shoot portraits on an old Mamiya twin lens (you can still see what you are getting when you press the shutter) and I have often put a card cut out over the ground glass to get a visual crop - but this is what works for me.
The main advantage with larger formats is you can see better what is going on in the frame and this improves your eye. When I was training I spent a whole year working on nothing but 5x4. When I picked up my 35 - after the initial shock of how tiny that viewfinder is - I discovered I could see a whole lot more in there and I could use it better.
A late night drinking session in the 80's with some of the UK's top pro photographers turned technical. It was agreed that what was shot on 10x8 for 48 sheet posters could be done just as well - and cheaper - on 35mm with acceptable quality (printing with half-tone covers a lot). The main reason we used 5x4/10x8 was that AD's expected it and it impressed the clients....
To sum up the disjointed ramblings of a senile old man: what are you going to use it for? Do you really need it? Find one that you like the feel of. Then eat with it, sleep with it and get closer to it than you have with any person (your partner will get jealous but hey!)
In terms of digital - I think the same applies. Whatever format you buy the CCD that captures the image is about the same size. It's all to do with the number of pixels used. What you are paying for is the glass and plastic around it and the name of course.
I can get a perfectly acceptable 10x8 out of my 4meg Ixus and it does what I want.
What are you going to use it for? Just how big are you going to print? Are you going to use it to take pictures or impress the neighbours?
Sorry if I bore. Tell me to shut up.....
I'll go quietly officer......