Druid charm spells

[QuasiGnome]

Hey guys, I have been charm killing pretty successfully (21 druid now) but I have a question about the spells I'm getting.

Is there a need to buy the new charm and fear animal spells? This is my first character here and animal killing means my funds can't cover many new spells. Would rather not have to buy the spells if they are not needed.

At 19 there is Terrorize Animal, but my current fear spell is still working fine for 1/3 the mana.

At 24 we get Charm Animals. Current charm seems to be good as long as I stick with charming blues. I get very few breaks.

Anyway, do these have some upside to them that makes them better than casting the lower level spells?

Any advice is appreciated.