I think you also have to take into consideration the speed at which you ar reaching the ECN's, and the validation of that order vs. a short list, buying power, and disaster recovery, etc. Simple ping times aren't good enough in this case, if you are looking for speed and client directed order routing. The client does have the power whether to self route or go out to a smart router, but any broker claiming that there isn't extra hop time to do this is not telling the full story.
You need to find a broker where you can self direct, and that has not only the speeds, but the ability to move tons of order flow, and doesn't limit the symbols you look at, the amount of symbols you can trade, or slow down the process in any way.
That's where it's more than just speed or just a generic "we have connectivity" broker. It's becoming a fairly specialized field, and some around here think a slower option is a good idea. I don't!
Some ECN's have better technology than others, too, clouding the rates at which you can access/trade them. Self directed would only help you exploit the speed differentials here as well....