Hi GTOz, interesting that you say there is not much difference in the image quality between the two, as posted originally my 550d IQ is quite staggeringly (to me - an amateur user) high. To be honest the more I read the more I wonder what I'm expecting, I mean take out the wide angle shots or to be fair the really really wide angle shots and your on a fairly level pegging, and when it comes to zoom shots then is the 550d more useful? I have been reading a lot recently and it all points towards the lenses you use, I have also spoken to two friends one a pro that shoots daily for a living and the other a semi pro and they also said that the lenses are the thing to concentrate on so maybe I should heed their advice! Thing is you get reading about the latest technology and convince yourself you need it or that it will make your shots look better, most cases the difference isnt enough to justify the costs I suppose especially as I already have a fairly high spec camera, all be it a "lower end" model.
I think what I will end up doing is buying a 5d as they are relatively cheap (£400-£500) and spend the rest on a canon L lens, I'd just be interested to know what the cost of shutter replacement would be on a 5d if anyone knows (uk price)