There's really no "shoulds", it all depends on your strategies. If you were successful trading the Q's then stay with the NQ, successful trading the SPY's then the ES is probably for you. If you've traded neither of those ETF's then put an NQ and ES chart side-by-side and see which of your strategies would work with each, the noise level of each, the rhythm of each, etc. Fills are generally easier on the NQ, liquidity is much deeper on the ES, each has advantages and disadvantages. P.S. There's been lots of discussions on just this subject at ET, if you do a little research I'm sure you'll come up with some good ideas.
Which one do you want to be trading in the long run? Why learn NQ if you are going to end up trading ES?...
I think YM moves around a lot more than ES... But does that mean it is more easily tradable than ES? I don't think so... Not for me...
YM in my experience trades with a one point spread about 70% of the time. When the market is moving fast, it can widen to 2-5 pts pretty easily. On balance, I think you can figure on an average slippage of about 2 pts, although a good number of your trades will only suffer 1 pt of slippage. So when scalping this gives you an advantage over ES and NQ, as 1 tick on the YM is only $5. YM also tends to have a little more carry-through momentum than NQ and ES on a very short-term timeframe.
The problem, of course, is that when you start trading the minis you don't know which one you'll end up trading. Or even if you'll end up with one as many people trade more than one. Some trader's rhythms (for lack of a better word) are better suited to the movements/noise of the NQ, some the ES, and some the YM. It's all about your strategies, and which of the indexes they will work on... assuming they will work on any.
It is easier to get a fill in YM if you are scalping because fewer people are lined up in front of you at any given price, especially when market range is very tight and choppy intraday.