I don't think you are interested in a cheaper but much, much slower ticket, but Eurostar will not reveal those to you. I believe that this Eurostar route does not use the same rolling stock (rather, the former Thalys trains ... ) as London-Brussels, so there won't be three choices of service, meals/snacks, etc. Antwerp has some excellent connections to Paris, so don't assume you have to start out in Brussels.
Another reason for going to Eurostar is that they might offer an "Any Belgian Station" add on (as they do from/to London), if you were coming from farther away than Antwerp. But it's not certain that even from Bruges, this would definitively be cheaper.
Just like with air travel, be sure you understand what an advance discount means for a sudden desire to refund, or change departure time.