Well I don't remember Throwdown, but Sudden Impact is MUCH better than Tapout.
You have 32 fighters to choose from. They left out Randy Couture and Frank Mir though :-(
The story mode is much better (There are two different "story modes" one is Champion's Road, which self-named, is a journey to the title of your choice, the other mode is more for a customized fighter training journey. Once you beat the Champion Road, you unlock a third story mode, Legend Mode, where you fight to be a legend.)
It's a fun game for UFC fans.