I use Canon, with Canon lenses or Sigma. You need to have a clear idea what your buget is, and then tailor your purchase to meet that. Don't forget second hand items can save you a lot of money and get you higher spec equipment for less money. I started with a Canon10D, which was an excellent camera to learn with, and was bought through ebay, at a significant saving. I was already Canon equiped anyway. If you're starting out get to handle the cameras we suggest at a shop. You may find one brand intuitively fits, as you handle it. I know a good selection of people who shoot primarily aircraft, and the bias is towards Canon equipment. Many use the 300/350/20D variants and frequently with the Canon 100-400 IS/USM. You'd need as a fag packet calc around £1500-£1800 beer tokens for new items. I would reccomend a second hand 20D, (with respect disregard HD's comments, they are not a current item, but they are in no conceivable way obselete). They are a very robust and good quality body, easy to learn, (two colleagues without any photo experience bought them, and got their heads round them very quickly). They are frequently used as back up camera's by pro's. 2nd hand you can get them for £5-600 from a shop, well worth a look. The 5D is superb for studio/landscape work but will be compromised for sport or action work due to its relatively low fps, as will any camera with about 3fps. That was one reason for changing for me. You will also lose on the 'benefit' of the crop factor which CtL mentions.
Your lens is another budget item to consider carefully. The better the quality optics the higher the price. The Canon 100-400 IS/USM can be had for around £1200 or £100.00 per month on interest free credit over one year. Beware the tie ins on 0% credit for the first year deals, on any gear though. Once you pass month twelve...
so pay it off in a year or get a bank loan!
IS/VR is not essential by any means. There is some assistance but good technique is an area frequently overlooked, and I'd dissagree with CtL's comments re speeds. None of my lenses are currently IS, and I've done sports, soccer, under floodlights too. So that may be another way to get a good quality optic, by not having IS/VR. You will however need to practise as well as having a degree of natural ability. Whichever brand you go for the Sigma 70-200 f2.8 EXHSM is worth looking at. You can increase the focal length with a 1.4 or 2x converter. That will have a loss of perfomance of the lens, but if you don't do much low light work, that will not be too much of an issue.
eg 1/15th sec hand held f2.8 400asa Canon 10D
1/125th hand held f14 200asa Canon 1DmkII
Search any of my image thread on here, they're all taken with Canon 10D/1DmkII and Canon and Sigma lens'