They sound like the same lens. Ask each seller what the Tamron model number is. Then compare to the price on the buy/sell forum here. I should have bought a reasonably priced used lens, but instead I bought the Tamron brand new. If I had stuck with the used market I probably could have found myself into the better Canon made version.
I've used this lens on a digital rebel for horse photography. It works well for the price, and if that's as much as you can afford then add it to your collection.
Like all entry level lenses, the focusing is a bit slow, in contrasty situations you'll get a lot of purple fringing and other artifacts, and it's not very sharp wide open or at the very extremes of its range. When shooting indoors it's not hard to run out of light because the aperture is 3.5-5.6.
At least for shooting horses, it's reassuring to know I don't have an expensive piece of glass on my camera when it's windy, dusty, etc. Shooting horses exposes me to tremendous dust and dirt.
