My subjective comment (and feelings):
F40 and F50: Both fantastic, outstanding "classics.
Handling: F50 much better, more stable, more reasurring, better (evolved) and different suspension system. For an average driver like myself probably more secure.
Power: F40 power delivery comes in a explosive way with the kick-in of the Turbos. That being probably felt that way because the F40 is lighter too. Weight differences mainly thru that great F50 12 cylinder ferrous block. In straight line comparison, the F40 will be quicker (weight advantage and torque difference due to different engine outlay), however when in life do we have straight line situations ?
Styling: Up to the beholder, F40 agressive 80's style, F50 smoother, rounder 90's style, both need some time to be cleaned afterwards
Sound: The Sound of the F40's wastegate is soooo beautiful, I'm missing that. However the feeling of driving a race-type of car is to me clearly more felt in the F50. Handling, direct vibrations, higher rpm's.....all towards the F50.
Seating: The F40 wins here clearly. IMO F40 seats are better than todays modern racing seats in street Ferraris.
Honest summary: They are different and both wonderful in their own means.
Suggestion: Try to drive both whenever possible and make your individual mind up.
Just my 2c.