If you have the money to pay the loss on the $50k house, you can do that and then keep your credit immaculate and buy your next house.
If you do not have the extra $30k and cannot borrow it from a 401(k) then you may want to keep the house and rent it out. Talk with a lender because you may have to rent it out for over a year before you can include the rental income to offset that mortgage. I know some people who have rented out their current house for a year while they rented the least expensive rental in order to allow them to buy a nicer larger house.
Speak with a lender or credit union first. I can recommend www.Amerisave.com/myrea/Alma or your credit union.
All the best,