On retail at least, as far as magic accuracy goes it's 1 skill = 1 macc, so that's simple at least.
For stats, it depends on what you're fighting. If you have at least 10~15 points more of the relevant stat than the mob has, it's also 1 stat = 1 macc. If the mob has stacked stats (read: it's a NM, or maybe an IT caster mob in an xp group) then you only get 0.5 macc per 1 stat.
EDIT: this was wrong, see below. Stats also improve the strength of the effect, which is why you typically see this kind of 10 stat vs 15 skill tradeoff on gear, but as mentioned sleep doesn't have a strength -- it's either active or it's not.
So yeah even in the best case it's 1=1=1 as far as macc goes, so as the others have said +15 skill beats +10 stats every time for Sleep.