in honesty... both are too powerful for a newbie (well the 500D is focused for Ameture Photographers.)
I like your post compared to other newbies who sign up to a forum and go "tell me a camera where i can get super High quality images for less than £500."
The 50D is for Semi Professionals.
IF you have the cash.. then I guess why not.
But I recommend buying a cheaper body and spending the money on Lens'
The most important thing for DSLR's are the Lenses tbh.
Ofcourse the body matters but its the glass that gives the true quality and captures the image (besides you pushing the buttons)
You should get a 350D to a 450D (includes 1000D which is essentially the 450D but cheaper and a lower resolution / MP)
As said above.. If you have the cash, why not go for the best of the lot (the 50D in this case)
*removed statement due to the correction below*
Whereas the 450D (thats what I use therefore all I can give a true opinion of performance on, the rest is just from what I have heard / researched.)
can take either EF or EF-s glass (meaning it can use L glass too) But as it is a 1.6x crop sensor all lens' you buy for the 450D will be focal length x 1.6
This means if you buy a 50mm f1.8 ii it is then essentially a 80mm (and also looses a little quality) but the quality loss is minimal
Remember: It isn't the camera that takes the picture, it's you!
In otherwords. You need to know how to control a camera to get good images there is no point going out buying a Canon EOS 1Ds mk II then keeping it in Auto mode (one of the best cameras in canons line btw)
Thats a waste of the money you paid for it, the reason DSLR's are so dear and popular compared to compact counter parts is because they allow for versatility thus making images more interesting and unique as you change every factor to fit the image, it's a type of witch craft
The only REAL advantage to the 500D over the 450D is the ability to record video, that isn't what a DSLR was made for, it is made for Still frame photography not movies, thats what you buy a Video Camera for (professional ones coming in at around 1.5k - 6k + which surprised me when i found them.. i was certain they would have cost more!)