For around the same price, I can get:
- Canon EOS 500D + EF18-55 + EF75-300 or
- Canon EOS 550D + EF-S 18-55 IS
Which of these camera combinations are better? The older 500D with two lenses or a newer 550D with lens that has Image Stability feature?
You haven't given any information about how these combinations will be used, so any answers have to make some pretty sweeping assumptions about that sort of thing. With that caveat in mind, though, here are some general thoughts.
If this kit is the only set of camera equipment you're ever going to purchase, and these are your only options, I'd have to give the nod to the two-lens kit because of the range it'll cover. I hope this isn't the case for you, though, because it's not a very good kit for most use cases.
Both camera bodies are solid options. Rebels are solid entry-level DSLRs, and both the 500 and 550 have capabilities that you could grow with for years. The 18-55 is also considered a decent kit lens. While often maligned as too slow or too soft or too cheaply-built, these lenses are really very suitable for someone who's learning photography. You'll eventually learn why photographers upgrade to more expensive lenses, and you'll (hopefully) be in a position to decide if and when those upgrades makes sense for you.
The 75-300 lens, on the other hand, doesn't have as much going for it. If this was the only inexpensive telephoto choice available for Canon, it would probably be a necessary evil, but Canon also makes a 55-250 lens that's generally considered to be sharper and better-performing than the 75-300, and you should be able to find one for just a few bucks more than the 75-300. One step further up the food chain, Canon's 70-300 (not to be confused with the 75-300) is fairly well-liked, and just beyond that, the 70-200 f/4 L lens is one of the more budget-friendly "L" (pro) lens offerings.
All things being equal, if you can pick up the 550 and 18-55 IS and save toward a more solid telephoto offering, I think you'll end up happier in the long run (and if you feed us a little more info, we might be able to help even more).
I had the 75-300 once. Terrible lens. I'd get the newer camera with the IS lens.
Have you considered one of those bodies-only and buying a standard 50mm f/1.8?
Depends what type of photo's you're aiming for really...