I have a couple and they are rather old Bushnells so I can't give advice on any specific makes as I've no idea what is current but based on my use here's a few things I found are important:
Trail cameras have a very limited range and usually work best at maybe 20 feet from the camera so they are no good at covering a large area but are better aimed at a known track or even at a baited area. They certainly won't cover a field at all well as the lens is wide angle so "far away" stuff gets very small in the image very quickly as distance increases. Also the illumination at night has a very limited ideal operating range - anything that is really close will just be overexposed and often somewhere about 40 feet is the maximum. Allowing for different models etc. you are probably looking at an effective working range from 10 feet to 40 feet, sometimes you'll get lucky but that's what you should expect and everything else is a bonus.
Trigger speed is much more important than image quality and megapixels. Some cameras are very slow to trigger and the quoted figure is often a daylight still image figure so they can be much slower at night and when shooting video. I don't know why, but that is just the way some of them work. You want to favour trigger speed over all the other features and to be sure that the quoted speed applies at night and also, if you'll want it, to video. Nothing will annoy you more than having lots of empty images where you think the animal has just walked out of frame before the camera triggered. Don't get carried away with image quality and megapixels as you don't want these photos framed on the wall, they are simply to show you what is present. I think Reconyx, who make what are recognised as the best cameras, are still at something like 3 megapixels!
The cameras need careful aiming if they are to detect animals passing - I found that the best height to mount the cameras for deer was about knee height with them pointing straight out parallel to the ground. If you aim the camera either slightly up or down then this can have a huge impact upon its ability to sense passing animals as the detector is pointed either at the sky or the ground and that reduces the area where a deer is likely to be detected - I once accidentally aimed my camera a bit upwards and had mounted it quite high on a tree beside a well used deer track, after over a month the only thing that had triggered it (despite fresh slots in the mud) was a passing bird.
Not all cameras have equal sensing abilities - some manufacturers are known for good sensing and if you put one of their cameras beside a camera with a less good sensor then the good one will have recorded a lot more images when you go back to collect them. Of course if you have a camera with bad sensing then you never know about it as you simply don't know about the images you've missed out on. When I bought my Bushnells I actually opted for models that were a year or two out of date as these had a reputation for much better sensing than the new ones at the time. It is worth doing some research to see what is getting a good reputation for sensing these days as you'll simply never know what you are missing out on.
The chasing game forum used to be a great resource for camera geeks but I don't know if it has much activity these days, worth a look:
