Anakela said: "Can you just post them for sale on both, and then delete the listing on the site with the higher markup/fees?"
I'm pretty sure Ticketmaster fees are higher across the board, but it has some advantages. For the seller, good placement and visibility on the official seat map on the official ticketing website. For the buyer, guaranteed legitimacy as the old barcodes are cancelled and new ones issued. StubHub's protection is generally well regarded, though.
Nothing's stopping you from listing on both but you better be quick to take down the other listing when it sells because if both sell you'll have a real mess on your hands.