I'll take UK kit in just about every department over the stuff I've used with the USMC & USN.
Helmets - yes they're light, but as has already been said, when you put NVGs on they float all over your head. You have to wear foamies underneath cause it's useless for noise attenuation. And when you drop the mask you have to attach it to your LSJ with a press stud on the end of one of the adjusting straps - so you have to re-adjust it every time you fly. Not a huge deal, but I just think the UK mask is more comfortable and convenient.
The US torso harness is a TOTAL nosebleed. You feel like you're trussed up like a gimp, and you can't just decide one day that you want to wear a jacket under it, or an immersion suit etc - it has to be tweaked every time the weather changes.
The survival aids in peacetime are about comparable. But when you go on ops, the US don't give you much more than a gun and a couple of maps - no CSW for them. Still, I was just glad to have a -112B with a battery in my LSJ by about week 3 of OIF.
I've flown on a bunch of US seats and apart from the F-18, they seem to be noticeably worse than the UK ones. The AV8B seat/parachute/harness combo has killed a couple of people who've pulled the handle in the heart of the envelope. Good old Martin-Baker at home has saved a number of guys who jumped well outside the envelope. And as for the seats in things like the T-2 and T-38.... You spend most of your time in the circuit in a T-38 outside the envelope. If you push any negative g in the T-2 you fly out of the seat, the dinghy pack flies up too and can get stuck so that you end up sitting on top of it half in and half out of the seat pan. They insist on blowing the canopy off in one piece, which needs airflow, so even though the seat may be zero-zero, it ends up 0-70 or something cause otherwise the canopy goes straight up and you do a 'Goose'. And while I'm ranting, the T-2 command eject handle has a position where: back seat pulls handle, both seats go but front seat pulls, nothing happens. And some instructors like to fly around like that ie front seater has no ability to eject either himself or both of you. Explain the logic behind that one...
The USMC don't even have dinghies with covers on yet - you end up in a WW2 style rubber ring. What's that all about? Especially when they're quite happy to transpac without an MPA.
Caveat all of the above with the fact I'm still in the US so everything in the UK is rose-tinted.