Hi,
Being new to this whole FFP thing and all, I have a dilemma and would love some advice. I don't travel much, (will be doing a bit more) but I have accumulated 8,952 miles on WorldPerks so far. In a month, I will be traveling from Den to KC, MO, and want to maximize the miles I earn.
(Also have 500 in Frontier, and 2 credits with SW -I live in Colorado)

The way it looks, with promos from United (online booking & check-in, and double miles for new MP members) I could earn a couple thousand flying with them, which seems to be almost too good to pass up ( and they seem to be consistently cheaper than NW). NW doesn't have much in promos right now that would help me, and I'm trying to find a good reason to stick with them, other than the fact that I have a bunch of miles sitting there that I can't seem to do anything with.
However, I am most likely to fly UA, Frontier or SW, since they seem to have consistenly low fares.

I also may stay in a hotel for 2 nights, and would need a rental car...

I would appreciate any advice that would point me in the right direction!
Thanks!