I agree with AKR, either camera will feel like a good upgrade, even the 40d with the same sensor as your 400d. I made the 400d to 40d step, and find the handlling, the ISO performance, the larger viewfinder, etc all a good step up. Personally I wouldn't want to go back to an XXXd body again. The extra savings over a 50d would allow more budget for other lenses. On the plus side for the 50d, the extra MP allow much more "croppability".