If you are going out to film, and plan to maintain your output 10 bit per channel I would advise the FxPlug version for FCP
And yes the more input frames per second you have, the easier it is for Twixtor, particularly for extreme slowmos.
Note FCP does not support frame rate conversion per se (input at some FPS and output at some other FPS) so you will have to do your slow-mo offline in you like and use a trick like an intermediate pass with QuickTime PRO to convert to still frames.
Pierre