Here's my thinking:
In an agro deck, I'm usually doing something to get my mage hurt: attacking into a guarding Iron Golem, walking into caltrops, taking attacks from a Gorgon Archer, getting set on fire, etc. Once I've taken even a small amount of damage, it seems much more efficient to heal 2 than to gain 1. Regrowth Belt will negate a Poison Gas Cloud, for example, while Sunfire Amulet won't be able to keep up. And if an Agro deck gets back down to 0 damage, it's probably winning already and doesn't need to go further.
In a control deck with other healing spells, though, Sunfire can be threatening. A Wizard with Sunfire Amulet and a Mage Staff of Heal is potentially really troublesome because once he stabilizes the game, he can heal himself better than he started.
It's also worth considering that regen from the belt can be used to remove bleed tokens, but life gain can't. On the other hand, if your opponent is doing lots of tainted damage, it's possible that you'll run out of vanilla damage to heal and it'll be more efficient to gain life instead.