OK, well I went to have a look at the D5100 and it looked great. Surprisingly small, noticeably more so than the 550d.
It's funny how all the differences that seem so large on the internet are almost entirely irrelevant or unnoticeable when you actually go and use it. I don't think you lot will be surprised by that though eh?
The screen differences are nowhere near as pronounced as I was expecting, and the menus are fine. Nothing wrong with that, despite my earlier slightly-embarrassing panic.
The guy in the shop, and my good friend, said the sensor was worse on the D5100 and the Canon 550d, with the focussing motor in-built, would be the better option.
However most things say the D5100's trump card is the sensor/image quality, so I will just put that down to the fact that he owns Canon's himself... the point about the lenses though is fair enough. Will I always be able to get AF-S lenses in the future though? Are all the semi-pro ones AF-s?
I only checked out the 18-55mm vs 18-105mm, and the extra zoom seemed to be very handy. I've been looking at the 70-300mm this morning, wondering if that's something worth considering, but I will probably consider it another day.
I've also managed to find it for £450 online in the UK, and with £50 cashback if you buy it with a lens, and £65 if you buy it with two lenses, it seems like a bit of a bargain at the moment.