I like both Ferrari 512's and both Porsche 917's - I think SMS did an outstanding job with the DLC cars and especially the Classic Le Mans track.
To drive in the game I slightly prefer the Ferrari.
In real life both had their problems - the power of the Ferrari was inconsistent with engines varying from 630 - 560bhp - some having problems with cylinder sealing (this is mentioned in Mark Donoghue's book). The Porsche had a flexible chassis which meant great care was needed when shifting gears to avoid missed shifts and engine damage (this is covered in David Hobbs book). Porsche had a better driver roster and the 917 received more development. Both were great and iconic cars and very welcome in the game.