I'm tool lazy to go read the spell descriptions on the wiki (as were you, so don't judge me
[You must be logged in to view images. Log in or Register.]), but it basically boils down to less mana/less duration vs. more mana/longer duration. Root is cheap (40 IIRC) but only lasts maybe 30 seconds max, while Paralyzing Earth costs more like 100 but can last up to two min or so.
If you know you need to keep several things rooted, but only briefly, Root is better. However personally I've found that I just always keep Paralyzing Earth memmed, because it's not that much more expensive and I often want to use it to "park" mobs as a form of CC.