Choosing between PhD students or postdocs as a lecturer I have been appointed as a lecturer in a UK university. For establishing my lab, the department head offered two fully-funded PhD students (research programme) or two postdocs. I am struggling with what to choose. I consulted with some colleagues but received contradictory advice.

Postdocs are more experienced for a starting lab but successful PhD students are better points for my promotion. On the other hand, PhD students have lots of official responsibility and paperwork while postdocs have no direct interaction with the university/department.
It obviously depends dramatically on your discipline and what you want to do with those positions.

That being said, in most cases I would assume that the decision should either be to hire 2 students, or 1 student and 1 postdoc. Here are some advantages of hiring students in your case:


As a new faculty member, you need to develop a profile for your team. PhD students are often largely a blank slate with not many preconceived ideas for what they will be working on in the next years - it's much easier to steer a team of 2 students into a coherent direction than 2 postdocs, who pretty much by definition already have a research agenda.
Similarly, postdocs should and will develop their research direction independently of you, which should also include letting them work on their own projects and publishing their outcomes without you. Mentoring postdocs to be fully independent is great and very fulfilling once you yourself are fully established, but in the beginning you may want to use your scarce startup funds in a way that better maximises them for your own career progression. 
PhD students will stay on longer. You have a better chance to develop them and publish extensively with them before they graduate. With a postdoc, it's easily possible that they are gone on to their own faculty position before you have really started to hit it off.
In many fields where some amount of manual labor is required, a team of 3 PhD holders but no students will be highly dysfunctional, with lots of talking heads and nobody doing the grunt work.
As you say, successfully mentoring PhD students will reflect positively on you in, or will even be required for, your promotion case. I don't think that having had postdocs usually really counts for anything in this regard.
You may simply not need a postdoc at this point. In my experience, postdocs are most productive in a large lab where the PI is stretched too thin to supervise all students effectively. This will not (yet) be the case for you.


Not all of those factors are true everywhere, but you should be able to decide for yourself what of the above is accurate for you. However, what I suggest avoiding is hiring postdocs in the hope that they will be some sort of pre-trained super-student, who will do as told with no real benefit to his own career. The few times I have seen young faculty take such a stance, it always ended with very little outcomes and a postdoc who quit after a short time.