The important points have been covered already, so I thought I'd give my personal perspective, as someone who sat both MAT and STEP relatively recently, and did BA+MMath at Cambridge,
The fact that most people make their Oxford maths offers compared to less than half of people making their Cambridge offers does not mean Oxford is easier to get in to. Cambridge invite basically all applicants to interview, whereas Oxford cuts about half of applicants before even reaching that stage (mostly using MAT scores). As a result, the two application systems have different focuses, but are of roughly equal difficulty.
The Oxford admissions process is relatively frontloaded. Getting the offer is very hard, but once you have it you're pretty much done. Most realistic applicants will have no difficulty making the A Level grades. Cambridge's process on the other hand is more spread out. It is comparatively easier to get an offer, but then you have to pass STEP, not an easy task. I guess that those more confident in their interview skills compared to exam skills might prefer Oxford's system (and vice versa), but I would personally suggest that this should not be a major factor in your decision. The two cities are quite different, and the courses have non-negligible differences too. These factors are far more important.
As for preparation, I would not say that MAT and STEP preparation are totally different. While the exams have different formats and emphasise slightly different skills, at the end of the day, practicing one will make you better at maths, which will make you better at both exams.
MAT is significantly easier than STEP, so I would practise that first. Do a practice paper at some point in the summer holidays to get an idea of how much work needs to be done. If you score highly (>80), probably just do the occasional paper, if you score lowly, do more. Up until sitting MAT at the end of October, it should be your main focus.
After sitting MAT, it's time to prepare for interviews. I personally did this by solving STEP problems, but just doing hard maths of any kind will help. Having a practice interview is very helpful if your school's maths department offers one. There are lots of myths about Oxbridge interviews, but maths ones at least are very straightforward. You sit down with some interviewers and solve problems (with their guidance). You won't get any stupid questions about your favourite fruit or anything, just maths.
The majority of candidates start seriously working on STEP in January, after offers have been sent out. Start with STEP 1, and move on to STEP 2 once you're ready. If your school doesn't offer help, there are tonnes of resources on the internet. TheStudentRoom has an annual thread that collates most of them.
If you want more specific advice/resources for MAT/STEP/interviews, I'd be happy to provide more.