Scion said:
if you can pick up that feat people were talking about a few days ago (boosts your effective level for animal companion) then definately upgrade to a higher level mount.
Unless you are thinking of a different feat, Natural Bond only increases your effective level up to a maximum of your total hit dice. It's completely worthless for a full druid. There isn't (as far as I know) any way for a full druid to get a companion higher on the list before he has the levels to do it.
However, there are a few others ways to improve a companion. One is to pick up the Celestial Companion feat from BoED, which adds the celestial template to your animal companion. You could also try to get your DM to allow you to train your companion and grant it the Warbeast template (MMII).
With a few mounted combat feats as well you can be a pretty incredible beasty
Even without the mounted combat feats, using your companion as a mount is very advantageous. It circumvents the poor mobility of a small character, and if you are ever attacked by a melee foe, you have a melee defender ready at hand (whom you can heal,
and heal yourself at the same time for the cost of a single spell.)
If you are not going to keep on riding (and that is also a viable option) then grab something that compliments whatever form you like to pick or that is best in whichever situation you need help with. Lions have pretty incredible full attacks with their pounce and grapples.
Very true.
Basically, the choice is between offense and defense. Getting a companion further down the list means getting a better offense and having a weaker defense. Keeping a "tier 1" companion means less offense and more defense.
Personally, I'd suggest that you go with a wolf (not dire), and have it pick up the Spring Attack chain as it gains feats (which it will quickly). By 10th-level, animal companions are pretty fragile. Even with good armor, low saves and few magic items means that they're easy pickin's for the baddies. At that point, a wolf with outstanding armor that can leap in--trip--and leap away again is going to be safer engaging melee foes, while still contributing meaningfully to the combat. On the opposite side is a big critter, like a dire tiger, that can leap in, do massive damage in its first round (pounce) and then get killed taking the return full attack. :/