While I know that most of you are joking, the amount of stereotypes and insults being thrown around this topic is disheartening, to say the least.
Both the UK and the US are wonderful places to live, and both have fantastic aspects as well as serious problems.
To be obvious, one of the main differences between the US and the UK is that the US is much, much larger. 300 million people spread over nearly 4 million square miles. A wealth of beautiful natural reserves and national parks dot the landscape, and there are many areas of the country where it's only wilderness for hundreds and hundreds of miles. Every climate type is represented within the physical spread of the country, from the Chihuahuan Desert in the southwest to the tropical areas of Florida and Hawaii to the bitter tundra of Alaska. If you're like me, you find all of this variety and wilderness refreshing and fascinating.
While the US also has dense areas, the UK is considerably more dense overall. It's a very crowded place, which creates problems in some areas. On the other hand, the small size of the commonwealth means that a big city (and the countryside) is never far away. Whereas it takes 30+ hours to drive the US north to south, the UK can be crossed in about 6.
As for the differences in people, it's simply ridiculous to say that all Americans are gun-owning, God-fearing, ignorant bozos, as some of you seem to suggest. There is tremendous diversity in this country, often to extremes, and it seems like every city or town you come across has a new perspective on the world to show you. Some areas are heavily religious (the Southeast) and some areas are fiercely liberal and largely reject religious customs (the East and West coasts).
The quality of secondary education (high schools) in the United States can vary wildly from region to region. Overall, though, US high schools allow students a flexibility and freedom of expression that I have not yet seen in most UK schools. As for universities, both the US and the UK boast some of the best in the world, but the US wins for sheer number of quality and variety of universities. Education IS outrageously expensive here though, no doubt about it, and that's a problem that needs fixing quickly.
Healthcare needs reform in both countries, though some of the healthcare problems in the US are downright shameful. Insurance companies are sharks, and an alarming number of Americans remain uncovered by the system. However, if you can pay for it, the quality and speed of healthcare and surgical procedures in the US is unmatched. Waiting lists and outdated equipment can be a problem in some areas of the UK.
As for obesity problems, both the US and the UK have very similar statistics. Both nations are having severe obesity problems, and I frankly think it's silly anyway for someone to avoid a nation because a certain percentage of people are overweight in it.
Both people in the UK and the US are very friendly and hospitable, with Americans perhaps being slightly more so. People in the US LOVE Britons, and the general view over here is that British citizens are all intelligent, cultured, and live idealistic lifestyles in a beautiful country. While it may not be the complete truth, you guys are pretty cool.
The UK and the US are hard to compare, and it's impossible to make a fully researched recommendation to anyone without having visited every region of each country. However, I can say that both countries provide a great standard of living, though both have problems that need to be sorted out.
Hope I was able to help out some.
Wherever you end up living, come by and visit El Paso, Texas sometime! It's not the best city ever, but the cultural experience here is fantastic, and the Mexican food is unbeatable.