Weed Eater One

My Weedeater One has been a constant disappointment. I have had one thing or another break every time I mow my lawn. It has proven incapable of mowing a half acre without something breaking. The latest issue is the drive chain comes off with the slightest bump. It is very easy to see why: The differential asm gear on which the chain rides is plastic. It flexes enough for the chain to come off and very predictably the teeth are wearing down. To replace the differential costs $300 just for the part and I doubt I can or should try to do it myself. It is out of warranty, so I am looking at $500+ to repair what was only an $800 mower new. Before deciding to throw more money at this machine I want to know if the new differential will be plastic too? There is no way I want to spend the money if the new part won't hold up any better than the original.