A couple of thoughts if it is not too late...
If you can, go to a store and try out both cameras. I was trying to decide between an XTi and a 20D and when I tried them out in a store I went with the 20D. My initial concern going to the store was "is the 20D worth the extra money". After trying them both out, there was no way I could get the XTi. I have big hands and the XTi was just too small for me. I had the same thinking as evryone else - it would be nice to have a smaller camera in my pack, but when i tried it out, I just couldn't hang on to it. I almost dropped it twice in the store. When I tried the 20D it just fit my hands and I could hold on tight and get a nice steady shot. Even with the size of the body, the first thing I did was buy a vertical grip for it to make it even easier to hold onto.
Function wise after getting the 20D I am glad I did. I used to be a landscape shooter only but now I use the speed of the 20D (the 30D is even faster) to get nice action shots - skiing, surfing, even birding. 5 fps of RAW shooting is amazing.
As for the Tamron 17-50 lens, I have heard good things about it IF you get a sharp one. It is a big IF. They have some serious quality control problems and I have read a lot of reports of bad lenses. I stick with Canon glass on a Canon body.
28mm on a 1.6x body is very restrictive for landscape photography - it is the equivalent of 45mm on a full frame 35mm camera which is close to 50mm which is what your eyes see. You really want to start around 17mm (28mm equiv) and get wider if you can. You need 15mm on a 1.6x body to get you to 24mm which is just great for landscapes.
If you are going to be just shooting landscapes while hiking, then one lens to think of is the Canon 10-22mm EF-S. It is an "L quality" lens and goes super wide. This buddha is 18' tall and I was only about 6' away from it (indoor, very low light, handheld, 1/8 sec! - yes, a good grip on the camera is important):
http://www.vftt.org/HI/070611-byodo-in/pages/1348-buddha-v-600.htm
Tim Seaver uses an XTi with the Canon 10-22mm on his long distance hikes. Small, light, powerful. Check out his posts to see some examples.
My advice in this situation is to not let the lens drive the camera selection though. You are better off picking the body you want and then getting the lenses you want. Even if you have to buy the body only and then the lens seperately and pay a little more.
So figure out what you want to shoot, how you want to use the camera, and see how they fit in your hands. Pick the body you want and then find the lenses. Be aware that you are not going to find one lens that does everything. That is why you are buying an SLR - so you can change lenses.
One other thing, I have never used butterfly photo. There are a LOT of shady camera dealers out there. You need to be very careful buying cameras online. I only do mail order with B&H Photo and Adorama. They are the hands down best. B&H has the XTi with the 17-85 IS for $1179 or the body alone for $662.
- darren
ps: 5 frames per second of RAW....AI servo, hold down the trigger and then go home and pick the best shot
http://www.vftt.org/HI/bodysurf/1714-800.jpg
http://www.vftt.org/HI/070526-POTN/images/IMG_1216-800-crop-v2.jpg