I had a Sigma 70-300 in Canon fitting, lent it someone....never got it back....still got the box though. I had already bought the Canon 70-300 IS lens which was overall slightly better, though I'm not sure it was better optically. Later I bought the 70-200 f4 L which is significantly better than either(and much more portable than the Canon 70-200 f2.8).
I've also got the Tamron 70-300 (Pentax fitting). I haven't looked up the history of the lenses but in my case the Sigma was bought new while the Tamron was secondhand, and quite a few years old, when I bought it. I can't quite directly compare the two. The Tamron was used mainly on the Pentax SF7, SFX and then the *ist DS.