Parking is free at both but easier at Scalliwags. You just park in the Fort shopping park car park. There is free parking at Fort Dunlop for FM, but you have to get a FM car park pass and take it back to your car and it's a bit of a trek. Upside is, once you have this pass you can use it again and again so only a pain the once.
They are both small and really aimed at slightly younger children but I find that the Food at Scalliwags is shockingly shit and there is a lot of useless space there compared to FM.
DO NOT GO to the Wacky Warehouse round the corner.