I'm a guardian at the moment and definitely feel more superior. The 25% health increase is really noticeable while the attack speed increase for zerker isn't. As well as the final skill, charging special attacks faster is absolutely inferior compared to a 60 or 50% damage and stun reduction. But that's just my opinion.
If you're fighting a boss in a team, you'd rather have the zerker than the guardian though - at some point, speed > damage. Also, the more hits you get in, the higher your damage and armor penetration goes. To be able to a hit an ogre, I'd need to get at least 5-6 hits in. According to Strippin who's been playing both zerker and guardian, the speed boost begins to kick in at 6+ hits. If you can stand at the sideline gettings 20+ hits in, I'm pretty sure you'd deal more damage than the guardian with tons of heal and damage+stun reduction. If you're soloing, yeah sure, the guardian kicks the zerker's ass. Just my thoughts on this.
Yah, I'll need to see more action on it.. hoping we can start having actual BOSSES scaled to the players online level...