The Radisson Blu is the only hotel that is truly connected to the airport and is accessed off the skywalk between the Station and T2 (it is possible to access T1 as well via the skywalks) it also has a carpark (at additional cost on what is not an inexpensive hotel anyway). The other hotels within the airport boundary are awkwardly located and will either need a taxi or a hotel transfer to any of the terminals, save the skywalks the area is not particulary pedestrian friendly. In that respect there are plenty of hotels near, but outside the perimeter of the airport that are better priced, if one is committed to a car transfer anyway.
I would drop the car off in central Manc find a hotel near one of the stations (there is a PI right next door to Picc) and get the train to the airport (strikes permitting) the following day and enjoy an evening in Manc.