yeah my approach so far has been: try 3.5 until i am disappointed in the results (hasn't failed me yet), once it does, pay for the upgraded version and see how that goes (totally will to, but 3.5 has really been great so far, and i was blown away when i asked about 1 equation that didn't seem right, and it did realize it's mistake and corrected it...i don't know...just seems more than adequate so far. i realize it's not perfect...i just haven't been let down sufficiently yet).