Nasdaq is all electronic where-as NYSE stocks use specialist firms which is why there may be a delay. On another board I read that a California based pensions company is taking several of the NYSE specialist firms to court because of the abuse of their positions. The sort of things they are accused of is front running orders, freezing the quote while they fill their own internal orders, failing to match orders with those already in the market and then taking the other sides themselves and even price manipulation.
So if you take that into consideration you can see why you get problems sometimes. It is funny because I only needed 1 experience with NYSE to decide it wasnt for me.
SPY is the stock that mirrors the ES and is hugely liquid. The advantage is that you can scale up or down with SPY where-as with ES you have to buy in complete contracts. The other advantage is that the spread is is a lot narrower which again is better for getting in and out of the market.
Paul