Out of Houston, with your employer choosing who you fly on, you're really going to struggle to do better than CO. You would always have to connect on NW or DL unless going to MSP/DTW/MEM/SLC/ATL/CVG. If you're happy to do that (or, more importantly, your employer is happy for you to do that), you might well get more upgrades, but it's likely the loss in convenience from not flying CO nonstops is going to be much bigger than getting occasional NW/DL upgrade out of IAH.

One special case where it might make sense to switch is if you mostly head east and would prefer to fly out of HOU. Then consider DL.