What an awesome opportunity to see two legends together. Out of that pair I am fonder of the F40, however there is no knocking an F50.
My holy grail of performance vehicle is the Mclaren F1 (the super rare LM more specifically). The F1 and F40 seem to be the standard for their respective decades, often compared as performance benchmarks. The Mclaren came into existence at the right time - modern regulations would have made for a very different car. For me it is the pinnacle and balance of design purity and uncompromised vehicle engineering - it was never intended to go racing, but with some tweaks it went on to take a 1,3,4,5 finish at the '95 24 Hours of Le Mans. Plus the 243 mph speed record it held onto for so many years...
Unfortunately they are a bit out of my price range, fetching $6-8 million nowadays depending on model/history.