I have only used nsi and I love it! I was previously using ASP and after switching to NSI it became clear to me the problem wasn't me ot was the products I was using. Nsi is really easy to use personally. But like i said I haven't used CND so can't comment on that side of it. I suggest getting some samples from each.
I have used both. Originally used CND then changed to NSI because of their range of coloured & glitter powders and gel. But after finding out I could mix my own using glitter & pigments I have gone back to CND and I am so pleased I have. I much prefer it and I find I can get better smile lines with CND.
They both have very different "workability" so I would get a sample of both and see which you prefer.
Used both but preferred CND mainly because I had a lifting issue with the NSI, once I changed to CND it improved then I learnt that I wasn't removing cuticle correctly.. so, I'm unsure whether the NSI would be better now but the improvement between the two was noticeable even without all the cuticle away.