I will suggest the 50D over 450D, but the xxxD series is already up to 600D, so even if you do want to buy the xxxD series, I suggest you buy the more updated models! There is also the 60D to choose from, but thats if you don't wish to upgrade to 7D or 5D or the 1D series, then you're better off with a 50D. The 50D is in comparison much more robust and better built then the xxxD series, comparing to the 12.1mega pixels of 450D, it is 15.1mega pixels, but then again the 600D is now 18mega pixels along with the 60D. The extra pixels does counts and works to your advantage if you're taking photographs of birds as in many case you will need to crop the photo. The frames per second is also much much faster in the 50D, even comparing to 600D. The 450D just have 3.5fps, while 50D does 6.3fps, it even scores better then the 60D which does 5.3fps.
I personally uses a 50D before I moved up to the 7D. I think the 50D is an excellent choice if you aim to be more serious in bird photography or other genre of photography in the future, it is fast enough to follow most of the action, responsive in most occasion and solidly built(I've dropped mine on the ground for at least 6 times and it's absolutely fine til this day!). It should be sufficient enough for all your needs. The only down side I have with the 50D is that the noise from the ISO can sometimes be an issue when using high ISO like ISO 800, sometimes even as low as ISO 500 I will get a lot of noise, especially when shooting with RAW, the noise becomes very apparent. But otherwise, I will recommend this camera. Hope this helps!
[ Last edited by kmatthew at 10/01/2012 12:11 ]
