I have 2 United Club passes my wife and I can use on an upcoming trip to London. Outbound, we have a 2-1/2 hour layover before departing ORD at 9:05 pm. Coming home, we have an early afternoon departure from LHR nonstop to IAH.

Should I use the passes at ORD, or save them for LHR?

LHR

LHR for sure. Way better club than ORD

Agreed. LHR no question.

Also never hurts to check EBay to see if anyone is selling close to expired ones for cheap last minute. Back when they were paper ones, I used to could get them for around $12.50 each for ones that expired in a month or less. Now they email them out to you (well most - some are still paper). I haven't bought any in awhile, but can't hurt to look. For 2.5 hour layover I would def pay $20 or less if I was able to get my hands on some.

LHR. I've been there twice in the past few months.

Better food, open (free) bar with good liquor. Showers and private toilets.

It is NOT the Polaris lounge, but closer to Polaris than the ORD United Club.

Are you flying business/polaris? If so, you don't need separate passes.