I'm afraid that 5 years is represents several generations of development in modern DSLRs just as it does with computers. That said, unless you are a professional making a living with your camera, you may be quite happy with a DSLR after 5 years so long as you are still getting the images you need and the camera is working reliably. I know more than a dozen photographers who are still happily snapping away with five year old Canon 20Ds or Nikon D70s. I'm not one of them...Until this week, my primary camera was 40D and I just took delivery a 7D this past Tuesday.
There are some notable differences between the 7D and 50D aside from the $700 premium and HD video capability. The build quality of the 7D is better...Both have magnesium alloy bodies but, the 7D has better weather seals all the way around and is a quieter camera in operation over all. It is every bit the pro to semi-pro level camera as it's main competitor, the Nikon D300s. Primary controls are largely the same between the 50D and 7D but, the 7D is a little more customizable. The 7D's mode dial lacks the basic "scene" modes of the Rebel and XXD series cameras. However, the 7D retains the"full-auto" and "creative auto " positions that work just as they do on the 50D. The 7D's new sensor offers marginally higher resolution but it also produces cleaner high ISO images than the 50D which was not expected...Most of us were worried Canon was just continuing a one-sided megapixel war while Nikon had rightly continued to focus on improved image quality. Continuous shooting speed is up 30% to 8 frames per second (matching the Nikon D300s that needs a batter grip to get there). And the other two big news items are a dramatically improved, 19-point AF system and larger viewfinder. Bottom line is this Canon should have introduced this camera when Nikon came out with the D300 in 2007.
The 40D/50D are largely the same camera. The big differences are the resolution of the rear LCD panel and imaging sensors. Canon was caught asleep at the wheel when Nikon introduced the D300. The 50D was Canon's attempt to keep 40D customers who were quite irked when their camera was basically rendered obsolete three days after it was introduced. Unfortunately, the 50D was rushed into production and represented a marginal improvement at best. That's not saying the 40D is a bad camera. I've shot one for two years and loved every minute of it. The $700 price difference between the 40D and Nikon D300, paid for my EF-S 10-22mm f/3.5-4.5 USM lens which is one of my two most used lenses. I've found the lens a lot more useful than I found the extra features of the other camera so I'm a happy camper and would recommend the 40D to anyone looking to upgrade from a Rebel XTi while maintaining a budget for better lenses. I honestly don't think you'd suffer a huge loss by going with the 40D versus the 50D. When the 50D came out, I still recommended the 40D because the 40D's price dropped to around $900 and it was a far faster and better built camera than any Rebel built before or since while offering better high ISO image quality than the 50D, even if it was lower in resolution.
Having said all this, the 7D is a major step up from either the 40D or 50D. But if you aren't already shooting top notch lenses, you cannot and will not reap the benefit of this camera. The only lenses I have that seem to showcase the camera's resolution are my 24-70mm f/2.8L and 70-200mm f/2.8L and 300mm f/2.8L, these lenses remain sharp but I can see the additional detail thi camera captures over my previous camera. On the other hand, my EFS lenses (10-22mm and 17-55mm f/2.8) don't appear quite as sharp with this camera as they do with my 40D.
So which one? Were I looking today, knowing what I know now and mindful of my budget, I'd still buy a 40D without hesitation. But if you've already got good glass (top notch EFS or L-Series EF) then splurge for the 7D. The 7D is very much a professional body in every way the 50D is not. But the 7D is the best APS-C format DSLR that Canon has ever made...You just need the lenses to take advantage of it.