I will try to share what I have experienced but if someone else comes along and says something different they are probably right. I would not try putting orders on Island during market hours. I have played around with ARCA and ISE as I post liquidity most of the time and wanted to lower my commission (i trade with IB with unbundled rates) I found that ISE was cheaper due to the higher rebate but my fills where 'bad' in that the market would often be four or five cents beyond my bid/ask. This happened about half the time and the other half the fills where good on ARCA my fills have been good and the cost lower than posting the limit order on NYSE. On average I think I save about 20 cents per hundred shares by using ARCA over NYSE. No price improvements though and I don't know how much that plays into the end result. Best of trading to you
the one thing that I was thinking was that, you'd rather route to ARCA because it seems to be that, if an order comes into NYSE and the special routes out (truely don't know why he routes out, but does) it seems more often than not to go to ARCA (I'm guessing because of the relationship between NYSE/ARCA)