In my experience, in all but a limited set of circumstances*, you will get an inferior fill when going to an ECN instead of NYSE during market hours. Most of the time, if you get a fill, it's because someone else who is better aware of the floor market took you out and flipped it to the floor for a quick scalp.
*Some circumstances are:
- "Fast" chaotic markets, like the time immediately after a delayed, gapped open or important news. You're still just guessing by going to the ECNs, but at least there are more people guessing along with you, and you're more likely to get what will turn out to have been a good fill.
- ETFs. That's why Island can't display QQQ quotes any more :-(
- Stocks with particularly annoying specialists, but seemingly only stocks that are neither too liquid, nor too illiquid. The more people the spec pisses off, the more action flows to the ECNs, just to spite him.