It will likely be a wash time wise, so really it's preference. I think 7:15 is a good time to be at the bus stop and assuming you don't have an abnormally long wait, you should be at the front gate before the 8:00am crowd are let in.
If you drive, you'll have better control over your departure time (no unknown bus wait) and being that early, you will be right up front of the lot, so no tram needed. But, you will need to monorail to MK, which is only about 5 - 10 minutes, but like the bus, you have to wait for it. In this scenario, I would still plan to leave resort about 7:15, that gets you to TTC around 7:30 and gives you a 1/2 hour to get to the gate, which should be more than enough.