I'm a Brit now living in the US and I've been teaching since 1994, first in UK, now in US. I did part of my first degree in the US as an exchange program, then had my US undergrad grades turned into British ones. Since moving here I've had my UK qualifications Americanized.
So -
Both countries obviously have regional variation. Unless you manage to find a truly horrendous school or your kids gets badly bullied or has a truly awful teacher, the single most important outcome for education is the parents and their attitude to education.
The UK system does rank 'higher' than the US right through until post-grad level. My A levels counted as US undergrad work, although my undergrad work didn't count towards an MA. A Masters degree is the same the world over. Many US high schools do AP classes (advanced placement) which most colleges in the US accept towards college credit, for the more advanced kids. UK schools are VERY unlikely to accept a US high school diploma as the equivalent of A Levels, and if your children wanted to go to UK colleges they would almost definitely need to have A Levels as well as any high school graduation.
Lower down the age range, schools just tend to make do and fill in gaps where there are any. The US has a different approach to how they do subjects so some elements of teaching, e.g. math classes and grammar, your kids may well be ahead. But at age 11 or so they will just be able to switch and may need some tutoring to catch up.
If you want your kids to go to UK colleges, you know about the 3 year rule, but international student fees are phenomenal - budget for it to be hundreds of thousands rather than tens (allowing for inflation, and who knows what rules there will be in 15 years?)
International Baccalaureate Schools are growing in popularity in the US. Several of our local high schools teach full IB alongside their traditional high school classes. IB is accepted just about anywhere in the world and seen as one of the best education systems available. It is flexible and respected, so worth finding out if any schools near you do it. It would be fine for a child to study IB until age 16 then switch systems. It would be harder to make a switch at age 17/18 though (for any system).
Wth the introduction of the common core the US is becoming more standardised in what it teaches. This makes it easier to compare, and the common core in the US reads very similarly to the national curriculum in the UK. They're both online if you really want to waste your time go into fine detail.
I actually think that I prefer the US system, but I'm in a very academic little bubble - big university town, with a wealth of high achievers.
About the US I prefer: no uniform, more kid-friendly, very positive attitude, kids seem generally more motivated, lots of extra activities for kids to do.
I don't like: education funded by local taxes, so closely linked to income levels, schools are big - kids can get literally and metaphorically lost in them, quite a lot of hype about how important the social stuff is.
Mixed feelings about: no setting/streaming until the last 2 years of high school, when AP classes come in.
Sorry that's so long!