Quote from EricP:
I would suggest you route via BATS for all thin stocks for best order execution performance on marketable orders. If it's a thick stock, then EDGX will be better, from my experience. I strongly suspect, but can't confirm, that some of the EDGX darkbooks will choose not to fill your order (which is their right), but then immediately take out your price in the market, which then removes the price that you wanted to fill against.
For thicker stocks, I don't see this (since there are many more shares available, front running your order isn't profitable). As a result, for thicker stocks, the lower fees and occasional price improvement advantages of EDGX outweigh the front running issue.
FWIW, I monitor execution performance on thousands of marketable orders per day, and have stats for average performance on each route. The suggestion above is based upon more than just a gut feel from watching fills.