As a tote-around lens, I'd say the 28-135 IS. I have the 70-200 f/4L and the 70-200 f/4L IS to boot. As far as the speed of the lens, you can't compare the two. The constant f/4 of the 70-200 is much better than the 3.5-5.6 so you really can't compare the two lenses; they're in a different class.
Another lens I would highly recommend you consider is the 28-105 f/3.5-4.5 II ($250). This was my primary tote lens prior to the 28-135 and in my opinion, may be a little better if you factor in the price; it's half the price! I regret purchasing the 135 to replace my 105 because it was not worth the difference in price for such minimal improvement. At longer focal lengths, I feel the 28-105 is superior to the 28-135; sharper images around the edges. At around 50mm, I feel the two are evenly matched, and at the wide, the 135 seems to be a little better, but not by much.
Really consider buying the 28-105 instead of the 28-135 if it's just a travel lens. The only downfall is the lack of IS on the 105, but if you don't shoot much lowlight, that shouldn't matter. Use that until you outgrow it--which you will the more you shoot-- then you can turnaround and sell the 105 on ebay for $180-$200 and use that money plus the $200 saved, toward the purchase of the 70-200 f/4L. <--- That lens, along with the 17-40 f/4L, and a prime 50mm lens is a perfect 1-2-3 combo for your camera bag; you'll have focal lengths from 28mm to 200mm all covered with nice heavy glass.
What it really boils down to though, is your finances and whether or not you intend to upgrade in the future. If upgrading and you can spend a few hundred dollars more down the line, go with the 28-105 and save $200 to apply toward your upgrade. But, if you plan on having this travel lens as your "be it all" lens, then definitely go with the 28-105; you'll appreciate the IS for lowlight situations.
And don't forget about a BG-E3 because the XTi body is a bit small. You'll notice that your right pinky gets tired after a while because of the lack of grip. And it also helps stabilize and balance out the weight of a large telephoto lens on the small body.
28-105 f/3.5-4.5 II USM:
http://photo.net/equipment/canon/28-105