It varies.
If you take a Dire Wolf when the level comes, it's a 6HD creature with Str 27 and humongous damage. If you stick to your wolf, it is a 6HD creature with (IIRC) Str 16 or so, small damage (compared to others), but several tricks and perks (Devotion and what-nots).
So if you're after combat power, you should trade in your animal companion for a larger one (or roleplay it as being a mystical evolution of your AnComp, like a wolf turned into a Dire Wolf, like I do IMC). If you want more versatility and less conspicous AnComp, stick to the basics and watch the bonuses accrue.