Both PolyArmour and NiCop work great, and are easy to bend and flare. You do need to secure them well to keep from being abraded.
I've done stainless, and won't do it again. I have a simple bar type flare tool, and even with tap magic, it took multple tries to get straight flares on it, and I had a few crack on me as well. The softer line flares much easier, and probably seals better. I speculate it would crush up against the fitting and seal more thoroughly.
There's no need for a check valve as has been stated.
I'm really not a fan of pre-bent lines, you pay quite a bit more for the "convenience", and wind up bending them anyway.