Sampras was stronger mentally. I would say he is stronger mentally than all of the current player: Fedalvoiv included.
Sampras arguably also had greater competition. A much bigger variety of players across different eras and accomplished surface specialists.
That being said, Nadal has just accomplished more. Period. The Spaniard has also lasted longer and is still NOT done.
So while it breaks my heart to say this, Nadal is the greater player.
Ultimately though what matters more is how we feel. Sampras - the humble champion and the king of swing - will remain in the hearts of his fans as the best. Always.
Pistol Pete Forever!!