If you didnt know this, Sleep II, unresisted, lasts a max of 90secs. Good to know this when you try to solo stuff and you want to know how long you got to rest. ^^
So, tonight we're fightin in Quicksand caves and two spiders link on us. I cast my newly acquired Sleep II (enfeeb capped) on it - resisted. Our Bard stepped in and used Foe Lullaby on it - resisted. Ok, what the heck, I'll try good old Sleep I - it stuck.
However, later I got to use it again and mob that was slept stayed out for the entire fight.
So with my cursory study, it would seem that Sleep II affects duration a lot more than the chance of sticking.
It is stronger than Sleep I therefore more reliable and seems to last a little longer. Also you have 2 sleep spells to use at a time instead of just 1. So, if ur sleep wears off before you are able to recast it again u can cast the other sleep spell you have...very useful and has saved me at least twice i can think of.