Bumping up the power of Animal Friendship isn't such a big concern, because most of the power of an animal companion comes from other spells, which the Druid has better access to anyway. Particularly with spells like Animal Growth and Nature's Avatar, nobody should ever underestimate the power of a good animal companion at high levels, particularly using all the stuff in Masters of the Wild. I worked up the stats on a fully buffed Legendary Horse for my (then 18th level) druid adventuring companion, and our GM hasn't let her have an animal companion since. We are all convinced that he is scared, he claims it have story considerations in mind. Basically, after a bunch of buffing spells (9 I think), the horse was a better fighter than anyone in the party. Scary.
If anyone wants, I can post those stats.
Basically, Rangers don't get access to the best animal buffing spells, and as such their companions won't be as powerful. And then people might want to play one again.
Personally, I'd go with level-3 for caster level.
--Seule