I'd use the cheaper one that lasts longer. By the time you can cast enveloping roots, ensnaring roots lasts 1:36 .... the other lasts a flat 60 seconds.
The benefit of Enveloping Roots is you have a better damage/mana ratio (not a big deal) and a
faster cast time (potentially a much bigger deal).
On my shaman I still end up using my level 14 spell:
http://wiki.project1999.com/Root
Reasonably fast cast time and dirt cheap on mana. Lasts less than a minute but it doesn't really matter much because my dots have a DD component that can randomly break roots.