It depends on the horse for me. If I have a broke horse coming in that has been ridden lots I will usually ride the first day. If the horse is young, needs training, or a problem horse I will usually spend some time on the ground.
Depends on what kind of horses you bought and what your goals are really.
Some horses are nutty when moved to a new place and need a day or 2 to cool down, but if she/he is rather calm,for the first few days I would do alot of ground work to see were his/her training is, how he/she acts/re-acts to things, and really just get to no the horse. Would do this before riding that way I would have respect/ better controll over the horse.