I find that really annoying, actually. There are many sizes and yet no guidance on which size to use when. I'm not even sure it matters, but it annoys me to simply have multiple sizes with no explanation. Jagwire puts out multiple sizes, and they offer no help or explanation at all.

I used to go out of my way to buy 5mm derailleur cable, just to be consistent with my 5mm brake cable. More recently though, I've gone to 4mm derailleur cable. It's easier to find.

FWIW, I never buy individual kits. I used to buy 25' foot rolls. More recently I've stepped up to 50 meter boxes of Jagwire L3 cable (4mm derailleur, 5mm brake). Once you get comfortable running new cable, it's really nice to be able to run a new cable whenever you feel like it.

Also FWIW, I've had good luck with Jagwire's stainless steel inner cables. I like the stainless. It seems slick enough. It doesn't corrode. And with the Jagwire cables I've had no trouble with post-installation cable-stretch.