Stick to Nikon and Canon. I wouldn't even go near Pentax and Sony, really (probably going to cop flack for this but it's what I know). As for the differences of them, errrr....ergonomics, software used, how you use them (to an extent). Basically they do the same thing in slightly different ways. A bit like Cider and Beer. There's a huge following behind both and you'll get
drunk a decent picture by end of it no matter which way you go.
The sensors come out of the same factory for both Canon and Nikon but I still swear that the tones coming out of the Nikon when both are shot under the same conditions are slightly richer - but nothing that can't be rectified in Photoshop afterwards and there honestly isn't much of a difference, just something that I have noticed. Also the APS-C sensors are rated at 1.6x for the Canon but weirdly 1.5x for the Nikon (meaning its slightly bigger and therefore better) but a lot of Canons these days are running 1.3x so you may be able to get one of those (not sure which models they come on though). If you want to learn more, just google '1.6X sensor crop vs full frame' (or something like that).

Not being a Nikon fanboy, just stating things as they are. I would say you get an awful lot of 'bang for your buck' with Canons these days.
If you are looking at any particular models, try searching to see if DigitalREV have done a review on youtube. They are really insightful on the technical bizniz and they usually do comparisons with other cameras in the same price bracket, so you really know what you can expect. 👍