I own both. I've had both since their respective v. 1 releases. To be honest, neither is perfect, and both have their own learning curves. I probably use Fluid Mask the most -- it's a little better when masking hair, etc., though I sometimes feel version 2 did a better job than version 3. Mask Pro, theoreticaly does a better job with transparent backgrounds (i.e. - glassware), though I've never really had a lot of success. A couple months ago I worked on a catalog with about 170+ silos and did 99% with Fluid Mask.
My suggestion: both come with 14-day trials. Download both and play with them and see which fits your work style the best and which seems more intuitive. I find either way, you have to do a certain amount of cleanup with the history brush to restore edges that have gone missing. Both have online learning videos that are worth watching.
Nemo