Canon 15-85 comparison with Tamron 28-75?

I have a very modest setup of 450D and 28-75 Tamron, a lens I love even though it's range is limited at both ends. Next May I'm going to be at Bright, (Victoria, Australia) an area that is really scenic and has beautiful autumn colour and I'm really tempted to get a wide angle for the scenery shots. First thoughts were Tamrons 17-50, which shoul be near as identical in IQ my 28-75, then I remembered Canons 15-85 which has what seems an ideal range or a walkaround.
In IQ, does the Tamron still have the edge, or are they about identical. There are lots of great photos on the samples page here of both, so any input from members who have lived with each would be appreciated.
(or, just to REALLY confuse things, I guess I could go FF instead and keep the 28-75, while hopefully getting a better camera into the bargain) Funds are pretty limited and the 15-85 would mean selling the 28-75, and even then it would be a bit of a stretch and preferably I'd get a 2nd hand lens to keep cost down.

All input most appreciated,

Hal55

I have both of them. The 15-85 is my always-on for the 60D. The 28-75 is my always-on for the 5D. I have used the 28-75 on the crop body when I need the speed, but it is not near wide enough. The 15-85 focuses faster and has IS. There is little reason to go to the 28-75 besides the f/2.8 aperture. If you are shooting scenery, f/2.8 is not much of a choice. I'd go with the 15-85. Keep the 28-75, though. It is nice on FF.

Thanks for the input - much appreciated. If I can pick up a 15-85 cheaply I'll get one, otherwise a 2nd hand 17-50 for about $200 seems a good enough option to take on the holiday. Can't really get enthused enough about replacing the camera, even though I'm sure there has been plenty of progress since the 450D came out.

Hal55

The new EFS 18-135 STM is supposed to be as good as (or even better than) the 15-85.
Might be worth checking it out.

I have the 18-135 STM for my SL1 and it is indeed a VERY good lens. In the past I have also used a 7D/15-85 combo and that was very, very good as well.

I think I would recommend the 15-85 for your 450D because while the 18-135 STM is a great lens, I don't think it is any sharper than the 15-85, and you really can't take full advantage of the STM focusing unless you have a T4i, T5i, 70D, or SL1 and plan to shoot a lot of video. (STM has better continuous video focus ability)

The 15-85 is a great lens on it's own, and the added focal length at 15mm is really desirable for landscape and wide angle photos.

They all sound good lenses but, being on a severe budget, the 17-50 is looking the likely winner simply because I can get a good 2nd hand example for around $200. The others wll be more and I can't justify spending extra. Still, I have some 4 months to see if a bargain turns up and will be actively checking ebay the whole time.
I also have a sneaking suspicion the Tamron 17-50 may be marginally the best lens, particularly for the landscape/autumn colour photography I'll be doing. Doubt there would be much in it though.

Hal55