UK vs US PhDs

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P.S reposting this here from a history thread

Hey everyone I wanted to start a thread about choosing between UK and US PhD programs. As many of us might be stuck between these very fortunate but equally distressing choices, I felt some advice from folks here might prove helpful.

Some of such questions that have crossed my mind so far: 

What are some of the major differences that one should consider while making a decision? 

Does a three year PhD from UK disadvantage you in the US?

I have been told that US programs typically prepare you more for teaching while in the UK one has to actively seek these opportunities? 

Are certain fields more popular in one place over the other? 

Does one make you better prepared for the job market than the other? 

Thanks!