My personal experience, and I understand everyone's is different. I went on a bender this weekend of grinding for a decent Imago Loop, since I'm a hand cannon afficianado and I've never had a good one to try out. So I hopped in the 320 strike list (where key drops are more common) and ground until I got a key, which was about 4 strikes / 40 minutes. Dropped out, and went to the Undying Mind 320 strike, ran it, popped the chest. Behold an Imago Loop, with a so-so roll of smallbore and Icarus. Did it again, this time it took 5 strikes to get a key. Mark of the Undying mind. Back to the strike list. First strike was 320 undying Mind, got a key. Got a trash Imago. Do another 5 strikes, get a key, got another MotUM. So I'm averaging a key about every 4-7 strikes, and a Loop about 50% of the time, with the class item occurring a tiny bit too frequently for my liking, but still acceptable. I also remember grinding for a Grasp last year, and getting one about every 20 strikes or so, without using the cheat method. Given that, I'll take the Key system over the blind grind. It doesn't seem as beneficial, and makes the grind feel even grindier. But in my experience thus far, it also seems to give me about double the chance at getting the strike specific item I want, plus gives me the added benefit of not having to run the same strike for 3 hours at a time for a single drop.