Which lens to buy for my DSLR?

I have a Nikon D 5100. After a bit of photography with the kit lens I have started looking out for zoom lenses and have zeroed onto two lenses, both Tamron. What i'm wondering is should i go for a dedicated zoom lens 70-300 mm or should i go for a more all purpose lens 18-270 mm. The reviews of both suggest that both are good, the 70-300 mm is cheaper ($400 after mail in rebate) while the 18-270 mm is for $500 (after mail in rebate). The reviews also indicate that the 70-300 mm is a little better in terms of border and centre sharpness and also better with chromatic aberration. But the advantage of the 18-270 mm lens is that you just need to carry one lens. What is your opinion on this? Is it worth sacrificing a bit of quality for ease of use (no changing of lens)?

The longer the zoom range is, the worse optically. All you get with a super zoom is flexibility. Primes are the best. Changing lenses is what the dSLR was made for. If I want to go light with only one lens, I bring a 50mm.

You only buy a third-party branded lens if you can't afford the same brand as your camera. Expect the third-party lens to be much more inferior.

For me it wouldn't be worth it. I can do most casual photography with a 18-100ish lens. I've yet to see a superzoom that comes close to a decent 70-300 in image quality and handling and focus.