I'm under mounting a sink to a slab of granite and there many different brands of epoxy. Is one better for this type of job?
Absolutely not. Epoxies are formulated for different cure times, cure temperatures, bonding to different materials, performance in various extreme environments, etc. That said, amongst the various hardware store 5 minute general purpose epoxies, I doubt there is a lot of difference between brands. I know nothing of the specifics for this application, I would expect there is a specific adhesive to use and recommended by the sink manufacturer. If not, I would check with the stone supplier for recommendations.
The common 2 part epoxies from the hardware store or home center are NOT waterproof. I learned this the hard way using them on boat projects. If you are going to use epoxy, it's worth the few extra bucks for a good one, like WEST System. You will probably also want some fillers like colloidal silica or glass micro beads to control the viscosity and sag of the final mix for an application like this.
Check out polyurethane adhesives like 3M 5200. They are made for use in continuously wet environments, and are generally easier to work with than 2 part epoxies. "Goes on smoothly, won't sag, and remains workable up to four hours. Cures in 5-7 days with no shrinking." Easier application and clean up than epoxies, too.