Tux in this case would be appropriate; one dresses for the end of the event rather than the beginning, so 6PM start means evening.
Between your two options, I would recommend SuitSupply. They make a better suit, so if you can find something on a rack at SS that fits, that's going to look better and last longer. If you have trouble finding a good fit there, I would recommend Kent Wang; it's a little more money, but the product is excellent. Like Indochino, but the quality is far, far superior. KW has a NY showroom where you can set up an appointment and talk about what you're looking for.