On the return leg of a STL-SMF-STL trip, I can book a RT ticket with a 12 hour layover at IAH. Alternatively, the same flights with 12 hour IAH layover booked as "multiple destinations" (STL-SMF-IAH-STL) prices out the same. Is there any advantage or disadvantage to ticketing one rather than the other?

Also, if booked as RT ticket, can I check-in additional baggage at IAH before my IAH-STL flight?