I own a CXS drill and I've been also thinking about T15 or T18 (or C15 and C18). I tried both "T" versions in a store (they did not have the "C"s in stock, but they are different from the Ts only in form-factor).
PROs of 15 Volt models:
1. Slightly lighter. However, the weight difference is marginal. You can tell the difference if you are doing a "blind test" and hold both drills in your left and right arm without knowing which is which, but it takes a few seconds of thinking before you become confident which is lighter.
2. Cheaper. This is kind of important.
PROs of the 18 Volts model:
1. Longer battery life. The battery may have the same capacity, but power = voltage * current, and capacity is just current * time. So 18 Volts versus 15 volts means 20% longer battery life, for the same power, or more power for the same time. This probably does not make much difference for practical purposes. 20% is too small to feel it easily.
2. Compatibility of batteries with other Festool cordless tools. They are all made 18 Volts these days. Extra two batteries are helpful to have at hand, if you are not using the drill but using, e.g., a cordless track saw, so you can have two batteries in charger while using the other two. This can be an important factor, but only IF you have other battery operated Festool tools, or plan to buy them.
3. 18 Volts seems to be current battery technology which is at its maturity, 15 Volts is more like yesterday's technology. This could have impact on availability or cost of replacement batteries when the originals die. However, this does not play a major role as Festool guarantees availability of spare parts for 10 years.
4. A little more power, but the difference is too small to be significant. Both have more power than one can use in a small package. If you need a lot of power and torque, get a corded 500+ Watts drill with a side handle.
So ultimately it all boils down to cost and ex-changeability of batteries with other cordless tools, I think.