Really both. I had the bike at a hair over 500 miles and ended up doing Appalachia on the stock rollers, & factory belt.
Later I did most of Montana, ID, & WA on the new belt and sliders.
Of note, I lacked a micrometer, but have a pretty darn good eye, & the factory belt was still good, but since I was there it just made sense to put in the new one.
By that time the rollers had been squared off due to consistent high speed running. I will note that the abuse I gave them was not the sort of thing the bike was designed for; however the sliders were still superior to how they performed new in my opinion. At 1k miles or so I'm still impressed with them.
I would have liked to try regular rollers a couple grams lighter, but they didn't arrive with the rest of the stuff I ordered so I didn't get a chance to do that.