There are some good promos right now for extra miles on several airlines, UA being one of them. Flights and hotels are definitely the most expensive parts of any vacation so I'd recommend looking at which one you need more of for your upcoming trips.
I know that I can turn MR points into airline tickets. I am not aware that you can turn airline miles into MR points. Obviously you can use MR points for hotel stays or you can turn them into Marriott Cheques, which = cash at Marriott hotels. When you use points for a night you do not get credit for that night. If you use Cheques you get credit for the night.