The last two trips with extended family we did both. We rented a car and used Magical Express (split the group up). Magical Express is so random as it depends on which resorts are on board, what time of day, how quickly the bus fills, etc.
The last time, my dad and I rented the car, drove to Wal-Mart, drove to Publix, drove back to the hotel and beat my mom, wife and daugter by literally 1 minute. They had checked us in and were on the bell hop van with luggage right ahead of us as we pulled into OKW. We just were able to beat them by parking right by the door.
So, our time to rent, drive and shop was equal to their time to bus and check-in.
We've also done ME to the resort and rented at the Hilton over by DtD (
Avis). That saved us the airport fees.
As a side note, and I don't know if this is always true, every time we've had a guest from Grand Floridian on our bus - they've ALWAYS been dropped off first. So, if you're at BLT this is to your advantage. If you're at AKV, this could be very disadvantageous. The bus driver (remember, they work for Mears and not Disney) did say they're instructed to drop Grand Floridian first no matter what. Then they naturally work their way around the park in best order.
We like the flexibility of a car, especially when at OKW, SSR, AKV, etc. Dining at resorts without a car can be tough. We haven't decided what to do for our split stay in October (BWV / BLT). For the BLT portion no car needed. For BWV... we have reservations at Jiko, Chef Mickey's (during BWV stay), etc. that make that more problematic.