I think the best stat to look at when comparing pitchers across different eras is ERA+. It reflects performance in relation to league average and is park adjusted. Where is Petey on the all-time ERA+ list? #1 by a wide margin (career ERA+ of 166, 2nd place is 148 - Lefty Grove). Where is Sandy on the all-time list? #31 with an ERA+ of 131. Petey's brilliance won't be matched for quite some time IMO.
EDIT: Because I love this topic when it comes up, take a look at their best year ever....
Pedro (2000) - 285 ERA+ (#2 all-time), .737 WHIP (#1 all-time), 5.31 H/9 (#4 all-time), 11.78 K/9 (#9 all-time)
Sandy (1966) - 190 ERA+ (#56t all-time), .985 WHIP (not in top-100), 6.72 H/9 (not in top-100), 8.83 K/9 (not in top-100)
All-time ranks for single season in those categories (which I think measure dominance better than any other stats).....
Pedro - #2 ERA+ (2000), #1 WHIP (2000), #4 H/9 (2000), #2 K/9 (1999)
Sandy - #56 ERA+ (1966), #17 WHIP (1965), #11 H/9 (1965), #40 K/9 (1965)
All-time ranks for career in those categories......
Pedro - #1 ERA+, #3 WHIP, #3 H/9, #3 K/9
Sandy - #31 ERA+, #21 WHIP, #2 H/9, #5 K/9
Petey wins.