The Oxford Maths interviews are no cakewalk; nor is their exam (which was particularly hard this year! *g*). STEP will look hard now, but from my point of view (nearing the end of Adv Highers/A2s) it's not bad at all. They threw questions at me in the Oxford interviews that were similar to the mock ones I'd given by someone who was
Go and visit both, if you possibly can - after going to Open Days at both, there was no contest for me and I picked Oxford, despite Cambridge's reputation (I've got an offer for Maths/Phil at Queen's).
Don't play it safe. If everyone played it safe, no-one would apply to Oxbridge. Apply for the place that inspires you the most - ultimately, you'll do far better there.