Thanks for the advice everybody. I think we are just going to end up using the UR points to get the airfare for nearly free. Its a shame that I won't be getting the best use out of those points, but I'd rather not spend the money.
As for the amount of time we are spending in Siem Reap, is it really too little? We are landing at 9 AM on the 14th, plan on relaxing and seeing the city on the 14th, going to the temples in the later afternoon. Then getting up super early to see more temples on the 15th before it gets crazy hot outside. We then plan on leaving sometime in the early afternoon on the 16th.
Would it be better to go to Chiang Mai? I am probably not going to be in Asia anytime soon after this trip. My wife and I are planning on having children soon, so this is likely our big vacation for at least a couple of years.