I don't use Osmocote for two reasons: 1) cacti and succulents grow so slowly, they need very little fertilizer anyway, and 2) I expect them to be in the same pot for many years and timed-release granules don't help after the first year anyway.

I just feed a couple times a year with a water-soluble fertilizer, and I water my C&S with acidified water.

Of the two, I would go with the 15-9-12.
I would use just a little, and then supplement with a soluble liquid fertilizer as Andy does. Your plants will grow quite a bit with regular nutrients. One of my friends fertilizes his Jades heavily, and they have put on incredible growth.

Josh