They are both wonderful ninjas but Kabuto should be gotten first as where you can study Tobi after since later down the Plot Instances or Elite Instances you get Tobi can't remember fully, plus Kabuto can line up pretty good with your team but you'll have to Use Main in position 1, Hinata in 2, Sasuke in 3 and Kabuto as last, this is the way they should be line up as you can get less out of the combo if I'm correct.

Not to familiar with the forum yet so excuse me if I did the picture thing wrong :D