Is a 3 year Ph.D. too short for MA/MSc graduate? Or put simply, 3-year vs. 5-year PhD?

Many programmes in UK (and EU) take up 3 years, as opposed to a minimum of 4-5 years in North America. For a masters degree holder, with a goal to stay in academia, is a 3-year Ph.D. too short to prepare one for postdocs and academic roles? Or would the extra year(s) in a 5-year programme be a repetition of the masters? 

Is the short duration enough in preparing an academic and his/her career? While the additional year(s) in a longer programme costly and dispensable? This question is informed by the many discussions here and elsewhere lamenting the competitiveness and difficulty of staying in academia, especially in the UK. I am asking from a UK/EU point of view (more specifically, social science), as I am not sure if this (potentially unwarranted) concern is generalizable in other countries and disciplines.
The thing about a PhD is that you should end up in roughly the same place. The length is therefore determined by where you come in. US PhDs tend to be longer than UK ones primarily because in the UK we specialise much earlier, so there is less left to do to reach PhD standard in your field.

In deciding what is best for you, the key factor will be you. How much do you still need to learn to complete your apprenticeship as an academic? That covers many aspects: knowledge for your research, research skills, how to write papers and give talks, personal management, career management, teaching, student supervision, probably some I've forgotten. If you have suitable funding, your PhD is a good time to learn these things while you have relatively little in the way of responsibilities. Also, at least in maths, your research record is generally considered relative to when you finish your PhD, not when you start.

My understanding is that the extra time in a US PhD is taken up completing taught modules/passing exams, not extra time on research. I would investigate whether having already done a masters would mean you could skip some/all of that and so shorten the total time.

Of course there are other factors to consider, finances and family often being the biggest ones. Also consider where you will be happy living, and who you want to work with. I was advised that the right supervisor is the most important thing in choosing a PhD. Personally I like to add that choice of supervisor should include preferred working patterns and personality, not just research topic, because you need to work closely with that person for a sustained period of time.