Keeping it makes more sense. Most cars lose almost half their value in the first 3 years. And even if you have a better-depreciating car, you're likely to lose more in the first 3 years...not just in terms of dollar value, but percentage-wise, than you are in years 3-6, or 6-9 and so on.
The best thing would be to get cars like 5-10 years old, hold on to them for a couple years, then move on. Those are still new-ish, and you can still find cars in perfect shape without much trouble. But the depreciation has slowed. You'll lose a lot less money that way, and you don't have to be driving beaters.
If you don't care so much about driving old cars, and are willing to do a little more maintenance, go with cars like 10-20 years old. As long as they stay in good shape, they're pretty much DONE depreciating. Once a car is between 1-2 grand, if you keep it in the same shape you bought it in, or better, you'll probably get what you paid for it when you're done.
Financially speaking, frequently buying new cars is one of the worst things you can do.