Which is a more accurate linear motion system?
I used a few different linear rail systems in my build:


plain smooth rods for Z motion
smooth rods bonded to a T-haped steel base for a 1200mm Y axis (similar to those at https://cdn.automationdirect.com/static/specs/suremotionlinear.pdf)
igus "low profile linear slide" for X (https://www.igus.com/drylin/profile-rail-guide)


They all seem to work pretty well. The one problem I had was with the igus slides, which I found had a little too much play -- so each change of direction on X had a slight backlash, from the "cars" twisting a tiny bit within the rails. I improved that with careful tuning: shifting the rails a tiny bit farther apart, so they kept the cars under a little tension against the inner sides of the rail -- some call this "preloading". 

I think systems that use actual bearings should generally be more accurate than spring-loaded slides. But the igus rails are still pretty good, and they're quite light, compact, and reasonably priced. I still use them, though once in a while I think about swapping them out to do a serious comparison.

The rails aren't the only factor in accuracy, though. I can't detect any play or warp in my Y rails, but that says nothing about accuracy and repeatability of motion along them. That's controlled by the motor and the belt, leadscrew, or other things actually moving them. Leadscrews, in particular, vary quite a bit in accuracy, depending on the shape of their threads, the kind of "nut" riding on them, and other factors. 

You can even make your own leadscrew system from just a threaded rod and a nut -- but those threads are not the same, and they allow far more play than a real leadscrew does. Nice article on the "backlash" problem at https://www.liutaiomottola.com/Tools/Backlash.htm.