I'm a Brit with a US green card and currently live in the US. I'll offer a very unscientific and purely subjective opinion to this thread.
Entering the UK pre-9/11 was great (from the US). No queue, a relatively friendly immigration official and off to collect my luggage. Post-9/11 hasn't made that much difference to this although the immigration officials seem to be less friendly. Fair enough.
Flying back to the US was also great at LHR, a quick check of my passport, 5 minutes going through security and 2 hours in the departure lounge waiting for my flight to be called.
Going through immigration in the US was a complete and utter pain and they were not friendly.
Now it's the opposite. I dread going through Heathrow immigration (after walking a mile or two to get there) and am then met by some soulless official who won't even return my "hello".
Flying back to the US (via ORD and/or LAX), I'm treated as a long-lost brother at immigration.
I'm sure everyone's experience is different and probably depends on what time they arrive, how busy the immigration official is, how badly their date went last night, etc., but my experience, like derekl, is that it's better in the US these days.
If they start showing me Disney films on arrival though, I may well change my mind!