Hi MeiHua,
As pboyles says, the ES has much larger volume and is therefore more liquid which is important if you trade size. But, just trading a few contracts it's not going to make much difference. Regarding fill time and slippage, all other things being equal (i.e. using same broker and trading platform etc.), you shouldn't notice any difference between the two. The benefits of trading the YM is that it's more granular than the ES, meaning that for every 1 point (4 ticks) movement of the ES you get a 10 tick movement in the YM. This really helps, especially on tight, rangebound days. Get a chart of each instrument side by side and watch them and you'll see what I mean.
I have always made a point of telling anyone who asks that no one market or instrument is easier to trade than another, merely that some traders prefer one market or instrument because, for them personally, they find it easier to trade. For me, the YM is an 'easier' instrument to trade than the ES and, IMO, is a more forgiving instrument for a novice futures trader to start on.
Tim.