Hi Dusty Bin,
It sounds like, to me, that you are simply trying to avoid being alone and not have a boyfriend at all - someone is better than no one and so you are trying to go for the 'safest option' rather than the 'right' option.
Forget everything, forget the fact that Mike may or may not go 'back home' in December (where is 'home', by the way?). You could stick with Mike and split up in a year or two. You could go back to Rob and it may not work.
What do you want NOW? Stop thinking about the future, the if's, but's and maybe's. Keep to the facts, to the present. Do you want to go out with Rob or Mike, regardless of what may or may not happen tomorrow, next week or next year. Nobody knows what the future holds.
You also have to think of Mike's feelings. I am sure if he knew you were seeing Rob and having these thoughts - he would make the decision for you.
You want to give it another go with Rob. Then do it. You want to try it with Mike then do it. Just be honest with them and with yourself.
I personally would let them both go. I am a firm believer that if you truely want to be with someone, then you would know straight away. You, I feel, just don't want to end up with nobody.
Be strong, make a decision, it may work, it may not. x