What would be the UK equivalent of a US PhD? A postdoc position? And in that situation would the stress be comparable? Or is the system just totally different?
My understanding is that a UK PhD will generally be weaker than a US PhD and would probably do a postdoc before being "equal", but by that point you really have to be looking at publication record to make a judgement.
I can't really speak for the level of stress in the UK system since I have never been part of it, but the candidates coming out of the UK system are in general weaker than those out of the USA (of course there is huge variation). I would put then 2 years behind the US graduates as far as research experience.
Well, it does take 2 years less! Most people hiring PhDs in academia are aware that UK/European PhDs are going to have fewer publications, etc, than their US counterparts. It may be a bit harder to jump straight to a prize fellowship from a European PhD, but it happens quite a bit.
In the UK at my institution, there was only optional teaching load for PhDs, you knew you had funding for 3 years before you started and there weren't many taught courses, so you can dive straight into research.
I understand this, but I am trying to answer the OP's question. The UK system puts a lot more emphasis in getting the student in and out in three years, but of course this means the students coming out the other end have less experience. There is really no right or wrong here, just differences.