Nice beefy legs you have there. Are you putting any other top support like crossbars between the legs? If so I would do pocket screws up through the crossbars. If not, I would definitely go with something that has more lateral support than the little right angle brackets you show, either a much beefier version of that, or heavy duty shelf supports would work nice. I don't think you need much to hold the top down, the structural integrity of the whole bench is more important.