I think that it depends on how you want to use it and how often you'll use it. The black lipstick from Strength looks like it's more opaque and like it has gold shimmer, so it will probably pull warmer. If you're looking for something that you can use to darken/ adjust other lipsticks as well as on its own, I think Jet would be the way to go. Personally, since I can never have enough odd colours, I'm planning on getting both.
Ironically, I could really use a grey lipstick for my Halloween costume and the new Jet would be perfect.
You ladies know I am only being rhetorical, of course we must have boths, my preciousess. If I just wanted to layer, I would get black lip mix.