I agree with other responses. I was an AA CK for six years and before and after that 1K for another dozen years. Even being ORD based taking elite-heavy flights I rarely missed an upgrade domestically and I was always able to upgrade my ORD/NRT flights (about six to eight times a year) from J to F using SWUs at booking. Back when AA was running three class 777s TPAC. I was matched to GS one year without asking when UA tried to pick off AA CKs during the merger chaos. I tried to fly UA a few times but could not get any global upgrades to clear anywhere near booking and I never did get a CPU to clear before 24 hours out even on elite-light flights. I just burned my regional upgrades and wrote off UA.

After retiring, most of my flying is between homes at ORD and RSW and UA has better frequency and pricing than AA. I buy P fares so I don't worry about upgrades. However, I have seen numerous times GS pax sitting in the back and they have top priority for upgrades. UA seems to have far more 1Ks than AA has EXPLs that you would be competing with for upgrades and most end up in the back on my flights. And I agree that there seem to be more GS at UA than there are CK at AA. So I agree with others who have posted that if the main reason for moving to UA is CPUs, you will likely be disappointed. But coming from DFW, where AA dominates, who knows.