orihime can heal more then nemu right now and chad/sado is a good wall for the vanguard spot and just remember there is only three healers in the game to
Just Get both. Its better to have a good healer heal a good tank. In the earilier stages of the game before people can attack assault line, You defense is very important and thats what Chado and Orihihime are for. Get Chad first as nemu will suffice til you get Orihime.