depends on what tray you have fitted to what style camera you use.
also what sort of use do you want to use it for. ie for actually backing up a drive/track or just seeing if kids are standing behind the ute.
some basics.... bigger the screen the better, go straight to 7" screen. screens that flip the image are better than having a camera that flips the image (so it looks like a rear view mirror). that gives you more camera options. muilitable inputs means you can run multiable cameras ie one short range, one long range. or connect dvd etc.
wired/wireless, wireless still require power and cabling is easy done on the utes and they pick up every other transmitter around to. so a bit pointless.
CMOS/CCD, CCD are generally far better than most CMOS but are a lot dearer. biggest thing is the amount of light they will work in. CMOS may not see into the shade very well.
color/black and white, color is better for reconissing objects but b/w work better in low light. most cameras will automaticly switch to b/w in low light.
resolution, most of the cheap CCD cameras are 420, still get 380's around. most really good ones are 550. but the sharper pics can be lost on such a small screen. one advantage of low res ones is they can work in low light better (for the same sized chip).
chip size, bigger is better. 1/3 is better than 1/4. the bigger it is the more light it can catch.
night vision, ie IR (infrared). try to avoid any cameras that have the led's behind the front glass with the camera. they often leak IR into the camera causing it to white out and you don't see anything. very much so when you have dust on the cover. some are good and mount the IR led's outside the camera away from the lens but remote mounted IR is best.
lens, most cameras come with wide angle lens (ie 2.80-3.8mm). great for seeing objects/kids close to the vehicle, ie within a few meters, but horrible to back the ute with. some places make cameras with tighter angled lens (6mm) which is much easier to see longer distance but you won't see the kids just behind the ute.
security cameras, reversing cameras are just board cameras just the same as a board security camera. so you can use most analogue security camera as a reversing camera. the good ones come with a lot more options so you can fine tune them better as well as being far higher quality.