I'm not sure why every one keeps talking about the spread like it's a cost. Even if it were then the YM would not be cheaper using that metric. With the YM you'll find your stop getting busted through, and often times the spread will widen.
The SandP 500 is the leading world index, so I assume that's why it's traded more.
The liquidity of the ES means that your stop will not be busted through, and during market hours it's very rare for the spread to widen. Anyway, if you use a limit order, then there is no spread at all. Even though the YM is 10 ticks per point, if the ES moves 1 point, the YM will typically also move 1 point or 10 ticks.