I will say again as I have said in other posts in other threads....
Friend owns a truck accessories place and says if you are going to make the jump go with 17s. Reason being is that if you look at the "stock" rim on most vehicles today you will find that 17s are stock for base model on almost all trucks and suvs. What this means is there is a higher demand for that size so you will likely be able to find more in that size for what you are looking for. As you can probably see, 15s (for a decent AT at least) are pretty scarce, 16s are starting to be that way too. Eventually, my friend says, there will only be a few offerings for 15s (less than now) and 16s will be like what we see now. Time frame, he thinks this will happen in the next 5yrs max.