ticketing as roundtrip vs multiple destinations
 

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?

No difference.

A 12-hour connection on a domestic itinerary gives you a fare break anyway, so no matter how you selected it, it's a multiple destinations itin.

You can most certainly check a bag (or additional bag) in IAH.

As long as you haven't already met your limit on checked bags. You don't get an extra allowance for the 12 hour connection.

I will not have any checked baggage SMF-IAH but will pick up 1-2 pieces of baggage for IAH-STL. Thanks for your advice. You guys are awesome! ^