Personally, I think you are better off with the oscar. With the midas, you are playing with a ticking time bomb from an aggression standpoin, imo.
Oscars grow fast, and are not easy fish to kill. They will not just sit back and let another fish target them, in my experience. Get the oscar a little bigger than the others... he's gonna outgrow them, faster, anyway.
Just my opinion.
-Rich