Re: a2 jazz vs a3 59?
The Jazz is a pretty thin and bright pickup and an Alnico 2 would make it even thinner and with less output. If you're looking for more fat and warmth, the only magnet that adds more bass than Alnico 5 is ceramic, which may sound harsh in an already bright pickup.
The 59 A3 sounds like a good idea, especially for a neck pickup. 59's are pretty mid-scooped hollow to my ear, due in part to Alnico 5's mid-scoop -- so an Alnico 3 would smooth that out fairly nicely though you'd lose a fair amount of output. Not an issue with a neck pickup but if you're talking about a bridge pickup, I recommend Alnico 4. It's a good compromise between 2 or 3 and 5. Decent output, more than 2 or 3, and much fuller and smoother in the mids than 5.