UK visas - student or fiance/marriage?

Hi,
I'm just after a little clarity in trying to reach a sensible decision; apologies if this query falls between two forum categories! My girlfriend is Peruvian, and has been made an unconditional offer to take a 3-year degree course at a UK university, starting in September 2011. She has (hopefully) all the requisite paperwork to be eligible for a student visa, and funds exist to pay the fees.

However, our long-term aim is for her to settle in the UK and for us to marry. I'm a UK citizen since birth, working, with sufficient funds to support us both. She is 30, we've been together verifiably for 3 years, she has visited the UK twice, and I've been to Peru twice. So, we would seem to be eligible to go the fiance/marriage visa route, which would get us a lot closer to fulfilling the long-term plans. Being a bit older, I'm rather more keen to get on with them.

Any views on which approach would be best? Would the fiance/marriage visa allow her to study? If she gets a student visa, would she later be able to apply for a fiance/marriage visa from within the UK, or would she need to return to Peru and start applying from scratch? And if she did, would the time she'd spent in the UK studying help her subsequent chances? Not sure I can face another 3-4 years of saying goodbye at airport departure gates!

Thanks in anticipation.