In your opinion, which would be the PhD project to take: i) A Position with a senior supervisor who's had many PhDs in an okay department of an okay university; or ii) A place with a young supervisor, who has only had one or two phds but who had published lots and is at a 5* rated department? Help!

who is offering more money?

Providing the younger supervisor is someone you can get along with (and has time for his/her current students) then you're probably better off with him/her. However, it may depend on your temperament, if you prefer a safer option then go with the more experienced supervisor who'll get you through the PhD, but might not have the reputation or the contacts that will help you get on after your PhD has finished.

Go with younger supervisor provided he/she is helpful as reputation and getting contacts is everything! The fact that the dept is 5* is a bonus as it shows excellent research of international standards is carried out there! It'll very helpful for you in the future!! You may also get to go to more conferences and have more opportunity to present/publish your work.

Kinda finished now passed with minor ed changes. Although my supervisor has a good publishing record, and was rather young 32 at the time, me being 31 then. He was really insecure about what was expected in a thesis and I wrote a huge volume. Which was cut down to half after the viva. Although I agree with the general comments, I just want to say that the younger student will mean a whole lot of more stress, imo.

younger supervisor of course ..

The younger one. I have an older supervisor and he is a nightmare. They are too set in their ways.

You need to bear in mind that a 5* department is much more likely to have better equipment and facilities, and provide you with the opportunities to network with other researchers internationally. Just because somebody is younger, it does not mean that their energy and communications skills are better than an older person! Have you been to visit both departments and meet the supervisors?

I've had interviews at both institutions, and I seemed to get on with the younger supervisor. I asked as I was concerned about their lack of experience with students. On balance I have decided to take the PhD with the younger guy, if I get the offers!