If you want it to be really strong, I would use 4x4 posts and "let in" (notch out) each post in the long direction of the table top to use with 2x6 for your top frame. So the 2x6 will sit in a notch in the post and be flush all around the outside. Shorter length on each end you could use structural brackets or beefy screws to attach, but in reality it isn't going anywhere. Once in place, you could carriage bolt them, or use timberlock structural screws or even some number 12 or number 14's. This way the load isn't being carried by the fasteners.

Hope this makes sense.