Both.
If I am feeling lazy, snap-on. Normally I have to flat iron near the roots after to make everything look seamless and smooth.
If I want it to look flawless, not have any incongruous straighter parts, occasional bends or lumps, etc. From the snap-on piece, clips.
I think snaps are better when you are far post or natural. I can make my curls spiral perfectly like they do with the clips if I use them with very thin pieces of hair, but the amount of time that takes isn't worth it for me unless I have a lot of NG.
I prefer using half as many rollers with clips otherwise. Also, I've been switching to plastic clips since the metal ones are get hot as fire.