The differences are small, but Superbuy still takes the leader role, by the looks of it.
Superbuy introduced the shipping expert service, not to be confused with the expert shopping. The shipping expert is a all-in-one consultant, rehearsal packaging and configuration service. Wegobuy has yet to offer a service like this, but none of the other competition has it either. It generally seems like new services are rolled out on Superbuy first, and then the rest of the market tries to follow suit.
Superbuy also has by far the biggest BBS bulletin board/forum, the other agents aren't really active in that format.
From my experience, coupon offerings are a more common occurence on Superbuy.
But Wegobuy has always had a slightly better exchange rate and overall prices. Superbuy is really a premium agent.
added: 6/26/2023