Here's my concern: the 16-35 (as well as 24-70) is a heavy lens that really throws off the balance of body/lens with a Rebel XTi. Since you haven't upgraded your body yet, why not start with a 20mm 2.8 prime? It's light, reasonably inexpensive, and fully full-frame compatible, giving it a long life in your camera bag. Used examples are easily found on eBay for under $250 - and its a very sharp lens. In any event, whatever lens you get, try to adapt to using a monopod at least for your travels. Small, light, and easy to carry, a monopod makes all the difference in your shots.