I switched from DL to CO last July (and managed to amass 100K by the end of the year). I chose CO over UA since most of my flights are on CO metal, and UA upgrade instruments were not valid on CO metal at the time. The programs will merge at the end of the year - I would therefore chose the program where status matching is easier if I were you.

My top priority is international upgrades. Both UA and CO beat DL hands down in this category - DL requires a high fare class to be eligible for an upgrade. CO requires a reasonable fare (W) for upgrade using SWUs and a co pay (that is credited back if the upgrade does not clear) if using miles from a low fare class.

International award availability: This is an endangered species on CO as much as it is on DL although CO's search engine is better (not as horrendous to be precise).

Domestic complimentary upgrades: 0/2 on UA metal, 10/11 on CO metal, 2/4 on UA intra Asia (NRT->ICN and NRT->TPE).

Shenanigans: So far I was upgraded after boarding 5 times on CO (2 domestic, 3 international including one time when the GA told me that BF boarded full just to follow me to my seat 15 minutes later)

Lounges: Nothing to write home about. I use the ANA lounges in NRT and the Asiana lounge in the main concourse (Vs the one in the satellite which is assigned to UA flights) at ICN.

Hope this helps