The Main difference is:
Z87M PLUS have only ONE PCIe 3.0/2.0 x16 Slot.
Whereas,
Z87 PLUS have TWO PCIe 3.0/2.0 x16 Slots.
It means Z87 PLUS Supports nVidia SLI Tech, whereas Z87M PLUS doesn't.
So, if you are planning to mount 2 nVidia GPUs, then go with Z87 PLUS.