Depending on what your goal is next year, once you make Elite. If you are going to travel international, then consider CO, or NW.
I like DL because I mostly fly domestics, and get auto upgrade almost 80%. I also like DL's award availability. Got HNL F 3 months aheads, and Int'l upgrade (using PMU) 2 months out. I can never use my CO miles to get award tix, but it could just be me.