seriously? if there was no reward for good grass players in 90s, how comes there were so many players with a game tailor made for grass, like ivanisevic, henman, philippoussis, rafter, krajicek, rusedski among others. if you say "no other time in history" that would imply this particular time in history is more rewarding for good grass players, yet there is not a single one grass specialist on top level.
in reality, sampras competed in much stronger competition and is, in my opinion, the best grass player ever.
I didn't say "No reward", I said "less". Still more than in many other sports.
The fact that noone now looks like a grass specialist is because being good on grass nowadays will also give you better results elsewhere. And the fact that the style of play which was successful on fast grass is less so on slow grass. So, as I said, if you feel the difference calls for both a "fast grass King" and a "slow grass King" then be my guest. If you choose just one, then Sampras is IMO no longer a contender, having been surpassed by Federer both in Wimbledon and in the grass warmups.