I didn't pick because of how well i liked them. they used to be my favorite characters(itachi still is) grimmjow went down the tunnel. Anyways, grimmjow may have the speed to outmatch itachi, but does not think plans through, and could easily fall for genjutsu and it's game over. Unless he could dodge tsukuyomi, encounter amaterasu with grand cero and fail, and fight susannoo in pantera form. Although susannoo is known for being the best defense, the down side, it consumes chraka from its owner/master in a couple of minutes depending on who is summoning it. By the time itachi will need susannoo, his chraka would be almost empty, and grimmjow out of energy, possibility of a draw is high.