The s40 and the 300 are both fine for any i5 even with a decent overclock.
As said the stock cooler is OK IMO, if you're after quietness & lower load temps though both those will max out at mid 50s with near silence.
They're good budget buys , I'd argue the gammaxx 300 is ever so slightly better.
Ah I see , the 6600k doesn't come with a cooler ?? Though it was just the 6700k that didn't.
Yes though the s40 is enough , I'd buy the gammaxx 300 though if its a similar price.