I suggest you just keep trading YM to get familiar ... then you can move to ES or NQ as you see fit. I don't see anything wrong with starting with YM, that's how I started (to lose money) and I moved to ES ... after switching from YM it may seem that ES moves slower ... as it thicker as apposed to YM. Good luck.
One tidbit that Carter pointed out is that because ES/YM trade on different exchanges, you can hedge yourself in the event the exchange that you have a position in has issues. So even if you trade one or the other exclusively, you might be able to put yourself in a neutral position in the event of technical difficulties. I've read of similiar cases of hedging (I recall Dennis of the Turtles fame did something like this with other commodities in a limit move), so I thought this might be useful to keep in the back of my mind.
another thing to look at over time is to consider trading the TF (mini Russell 2000)...reason being is much more pop $$$ per contract and therefore less $$ spent on commissions...just a thought
if you dont trade the ym but just look at the charts, you may get the impression that your stops would get hit less, but real experience of trading both the ym and es side by side would soon show you that the es chart prints are much closer to reality
NQ is a really nice alternative to ES or YM that most people do not think about. It is smoother then ES, and trades more volume then YM, making it the perfect US index out of all of them IMO.
overall, it looks like the NQ is bullish now, the ES is bearish and the YM in between currently...but for sure, the NQ is leading...