I've never owned either of them but I'm seriously considering buying the 70-300mm. It's cheaper than the 18-300mm, cheap enough in fact that you could buy that Tamron 17-50mm with the change. It'll work on full frame if you upgrade your camera body one day but in the meantime you'll get images that are sharp corner to corner with pretty much no vignetting.
Another much cheaper option might be the 55-300mm. I have the 55-200mm at the moment which is very similar. The 55-200mm is certainly capable of producing some very nice images. The problem with these two though is that they feel cheap and plasticky next to the more expensive lenses. It seems unlikely that they would last as long.