A metal spur gear on a stock maxx cb won't last very long. I ran the RRP spur on my maxx with the stock 20T cb and it chewed it up in one weekend. Then I got an RRP cb and didn't have wear problems anymore.
I only ran metal on my 21 maxx. On my 18/2.5 chassis maxx, I always ran plastic and I'd run kimbrough spurs. They seemed to hold up longer and get teeth chipped off less often. I'd easily make it through a few gallons before messing up a spur. Normally, the only time I'd chew one up is when an engine mount cracked or engine mount bolts rattled out.
Stick with plastic. Cheap week point in the drive train. It's easier to swap out a spur than it is to rebuild a tranny or diff.