If a camera feels a lot more comfortable for you, then get it.
Comfort factor means that you will be more likely to take it with you.
If you cannot afford the 15-85mm now, then you have two options.
1) Save up a little while longer till you can get it.
2) Get something else instead.
If you have a budget, stick with it. Going over budget is not advisable as there is always another option that is just a little more expensive. I have fallen for this a few times myself.
I enjoyed my old 17-85mm while I had it. The 18-135mm looks to be a nice lens itself.
Do you want a longer range or a faster focus?
Answer that question, and you will know what lens you want.
If you cannot answer that, then pick the cheaper one.
Chances are, whatever lens you get first will be replaced later on once you find out you enjoy photography.
If you find you do not enjoy photography, then you will have saved yourself some money.
Later on, if you are enjoying whatever lens you get, then you might want to consider trying a 50mm prime.
The 50mm f/1.8 is cheap and gives great results, while the 50mm f/1.4 USM gives slightly better results, faster focusing, better build quality, and more light intake for three times the cost.
Another option is to just rent either the body and the lenses, or buy the body and rent some lenses till you decide which one you want.
Canon 5DIII, Rebel XTi/400D
Canon EF 16-35mm f/2.8L II, 24-105mm f/4L, 50mm f/1.4, 70-300mm f/4-5.6 DO, 85mm f/1.8
Sigma 150mm F2.8 EX Macro
Sigma 10-20mm f/4-5.6 EX, Canon EF-S 18-55mm f/3.5-5.6 II
Canon 430EX II
Opteka 13mm, 21mm, and 31mm extension tubes
Vivitar 50mm f/1.8 for OM System