I was in the same situation a while back, so here goes.
1) Go to whichever gives you better funding.
2) The Cambridge MPhil is much more IR than Comparative, so I'd have a look at the course offerings.
3) Both are well-regarded, at least for UK PhD/DPhil admissions. The Oxford DPhil admits more from its own MPhil than Cambridge does, if you are planning to continue your studies in the UK system.
Feel free to PM me if you have more questions.