If you're flying on the lowest fares the 750 minimum on DL will yield only 375 MQM -- not 750 MQMs.
I see that there are 7 daily MLB-ATL flights. 4 are RJs and 3 are MD80s.
If upgrading is a priority, if you aren't getting upgraded regularly now, and upgrades make it worthwhile flying out of MCO, by all means do so!
If you'll be required to fly US when headed to PIT and PHL, and you do so often, perhaps you should stick with US. It would be better than splitting your miles across two carriers, IMHO.
If you can get around that restriction, consider NW. You can credit your MLB-ATL flights on DL to NW and earn full mileage credit even on the lowest fares. And upgrades on NW metal are pretty good for NW elites.