We don't find the monorail to be that convenient to get to MK. However, I will say that when we stay at GF, we usually take the boat over to MK, and that is convenient because a lot of people don't seem to realize that it's there so lines are usually shorter.
But I have to second the suggestion of WL, especially if you already need two rooms. We found it very convenient to take the bus or boat from WL to MK because it gets you to the bus terminal, where all the buses to other resorts are, so it gives you lots of options. You can do that at WL, too, but WL is less expensive, and I think it has nicer grounds and a pool.
We usually pick our resort based on where we plan to spend most of our time. If it's HS/Epcot, we stay in the BW area. If it's MK, then usually WL (sometimes GF, and we've done CR once - not our favourite - and we're trying Poly for our upcoming trip, but only because we got a good deal). If it's AK, then AKL. So as much as we love BC and their pool, if we were spending most of our time at MK, I'd do WL or GF. If you are planning equal time in all parks, then it's a slightly harder decision. If that is our plan, we will usually determine which park we would want to be at the latest at night and then pick a resort that's closest to that park. Late at night, I just want to get back to my room as quickly and easily as possible.