Assuming you only care about the special rolls, the strictly optimal thing to do would be to note how often Reaching makes a difference in getting a spot vs not getting a spot, and if it makes a difference more than 1 out of 3 times, go for it, otherwise Generous. 2 spots with 6 rolls each is equal to 3 spots with 4 rolls each, but if you get, like, 90% of spots without Reaching then 9 spots with 6 rolls each is significantly better than 10 with 4.
Personally, of course, I'm lazy so I'd just go with whatever enchantment popped up first until I had every tool enchanted and maybe a few weapons as well before I started rerolling anything.