You mean the Tamron 27-75 vs the Tamron 17-
50 f/2.8? (you say it right later in your post, but not in the poll)Definately
the 17-50 f/2.8 (as long as it is NOT the VC version!)
The 27-75 lacks the wide angle range for crop cameras (as you will know from shooting with your kit lens). The Tamron 17-50 f/2.8 non-VC, however, is considered to be one of the sharpest zoom lenses for that low a price
, with such a big aperture. Generally people are very sceptical of third-party lenses, but the Tamron 17-50 f/2.8 is well-known for its budget sharpness. The VC (image stabilisation) version of that lens, the 17-50 f/2.8 VC, does not have that sharpness (strong haloes, soft edges).
I have the 17-50 f/2.8 myself as well for my 60D, and I like it a lot. Alternatively I'd have got the Canon 17-55 f/2.8 with IS, which has slightly longer zoom range and image stabilization and a tad more sharpness... but I would have had to pay more than 2½ times as much.
The 17-50 f/2.8 is relatively light sensitive, and very sharp image-wise. The biggest downside is that its zoom ring turns the opposite way than the canon's (which I get used to rather quickly), and that the autofocus motor is noisy
. However, it's decently fast. If those two things don't bother you, this is the best zoom lens you can get for that money, with the right reach for a crop-camera. On a bigger budget, consider the Canon 17-55 f/2.8 IS.
This picture was taken with the 17-50 at 50mm, f/2.8 in a not too brightly lit room. Nothing fancy, have only just got my new camera and lens myself.