I have been using MPI's one piece chrome moly trailing arms since 98 (two set's of arm's, one on a XTC the other on my Viper. The lighter weight is nice. But what you'll notice most, is the round tube design allows deep snow to funnel out between the belly pan and trailing better for less snow build up in the front end.
As far as durability goes, no issue's with the original set I bought for the XTC. I sold them to my buddy when I sold the sled. He is still using them on his Mountain Max today with no problems.
The set I have on the Viper, I've had some cracking issue's at some welds, ( the welds are not as nice as the ones on the 98 arms). I pulled the arms off and took them to a local machine shop, and had them TIG welded up. They seem to be holding up now.
Not sure why there is a difference in weld quailty between the two set's. I bought the Viper arms in 02, so they were some of the first ones made for the Viper, maybe they had a different welder working then.
I finally bent one this year ( hooked a carbide on grated bridge), it put about a 3" wow in it. Best thing though is the arm bent without damage to the tunnel. I was about to replace it when I figured I might as well try and straighten it. Holding it in a vise between a couple blocks of wood, and using length of exhaust tubing (soft so it dosn't kink the arm), I was able to get it PERFECTLY straight again! Something that is not possible with stock arm's.
Aftermarket arms use the same part # for the Mountain Viper and short track's.