It is hard to say whether one is better than the other based on cleanup ability or longevity. What you want, I assume, is the ability to plug a hole? The harder-to-clean-up one (Slime) can be said to be more tenacious, sticker, more reactive, and quicker sealing and that is why it last only 2 years.
I am just saying there are different ways to interpret data. Maybe Flat-Out is better. I don't have a personal preference. But easier to clean up and life in an inner tube does not imply Flat Out is better.
I am not related financially professionally or in any other way to either company.