I think that a raft is easier to do , you need far less co-ordination of trades. For a raft you need someone to level and fill the site then hand it over to a team who will shutter the perimeter lay the membrane, tie the steel and pour the slab, whereas in a strip found you must dig the trench, hardcore the base, tie the steel , pour the strip, build the rising walls, fill the floor slab, lay down the membrane lay the mesh and pour the slab. You have different teams doing different jobs and you could lose time waiting for one team. Costwise it is really only concrete and shuttering that should be extra but you will be saving on the blockwork and some digging. And i think it is a better job ( certainly in bad conditions, and perhaps in yours if you are on ock already it really could be a pain digging in solid ground)