We've used National and Alamo at MCO. Both were great. Definitely get the free emerald club number for National or *********** number for Alamo. For National, just go get your car. No line. For Alamo, do the save time thingy on their website and sign in your dl number, etc. Then go to the kiosk instead of the line, fill in the info and head to the lot to get the car. No lines. Wonderful!
As for car seat, I'd never rent one from a car rental agency. We have a baby equipment rental business in Ottawa and there are a few in Orlando as well. We brought our seats with us when we travelled in Jan but I'd rent from a private agency before the car rental places. Private agencies clean, sterilyze and do safety checks on the seats. From my experiences, car rental agencies do none of that. Further, most won't guarantee availability of the seats on arrival as its first come first serve. You may be out of luck. If you book from a private agency, they'll likely deliver to the airport for you and they'll be new, clean and guaranteed ready to go. The cost is also usually about teh same or cheaper than from the car rental agency.
HTH