Cossa,
Yes, I believe there is a difference in the two spells.
Fetter:
Spell Type: Detrimental Skill: Alteration
Mana: 75 Target Type: Single
Casting Time: 1.75 Duration: 3.0 mins
Recast Time: 2 Resist: Magic (0)
Fizzle Time: 2.25 Range to target: 200
AE Radius: 0 Interruptable: Yes
Location: Any Time of Day: Any
Reflectable: Yes
Greater Fetter:
Spell Type: Detrimental Skill: Alteration
Mana: 140 Target Type: Single
Casting Time: 1.75 Duration: 3.0 mins
Recast Time: 2 Resist: Magic (-30)
Fizzle Time: 2.25 Range to target: 200
AE Radius: 0 Interruptable: Yes
Location: Any Time of Day: Any
Reflectable: Yes
For an additional 65 mana you can use Greater Fetter and get -30 to majic debuff on mob. To me that is like the equivelant of a minor tash built into this root spell (lowering the resistance to the root.) So hell yeah, more often than not when you use root you're in an emergency situation anyway, I personally will spend the extra mana to give root a better chance to land and hold the mob at bay.
Just my 2 cents.