Fedorov was decently better defensively than Forsberg, who was decently offensively better than Fedorov. Both players were roughly equal in being playoff monsters.
The difference would seem to lie in how long they maintained the level of play that they are known/heralded for, and I think that goes to Forsberg. Even if he was injured often, he was still "Forsberg" for longer than Fedorov was "Fedorov", at least in the regular season. Forsberg was one of the best players in the league for arguably ten straight seasons - Fedorov was in his prime element for perhaps three?
There's a strange valley in Fedorov's career from '96-'97 to '98-99 during which his offensive production is fairly significantly lower than it had better prior and considerably lower than it would be again from '99-'00 to '02-'03.
I think it may have gone largely unnoticed at the time because it coincides with the Red Wings' first two Stanley Cup wins of the Yzerman/Fedorov era and because Fedorov still played a prominent role in those two Cup victories.
However, Fedorov went from having scored 277 points in 202 (1.37 points per game) games in a span of three seasons to scoring 143 points in 172 games in three seasons (0.83).
That's a decrease of 0.54 points-per-game (more than a half point per game)!
This wasn't entirely due to the onset of the dead puck era, either. Even if we compare his adjusted point totals from those his two highest-scoring seasons ('94 and '96) to that latter block of three consecutive seasons, the difference is quite significant:
He goes from 109 + 102 (211) adjusted points in 160 games (
1.32 points-per-game) to 65 + 20 (in 21 games) + 72 (157) adjusted points in 172 games (
0.91 points per game).
In terms of adjusted points per game, Fedorov suffers a
0.41 points-per-game decrease between his 1994 and 1996 seasons and his '97, '98 and '99 seasons.
During this period, he also only finishes ninth in award voting once (he placed ninth in Selke voting in '97) whereas he had previously won the Selke and placed fifth in Hart voting in '96, had placed fourth in Selke voting in '95, had won the Hart and Selke in '94, had finished fourth in Selke voting in '93 and had been the Selke runner-up in '92.
Fedorov bounced back from this odd lull in his career by averaging 0.93 or
1.03 adjusted points-per-game from '99-'00 to '02-'03. He also again received Selke votes again consistently during this period, placing 12th, 8th, 8th and 19th in Selke voting. Additionally, he placed ninth and 19th in Hart voting during this phase of his career.
Overall, Fedorov had an incredibly elite couple of seasons (1994 and 1996), and if we include the lockout-shortened '95 season, we can say he was a world class player for three seasons.
Additionally, he had a string of four consecutive seasons where he performed at the level of an elite two-way forward from 00 to '03. However, during this latter stretch, he still wasn't quite at his peak form.
In between these two stretches, however, he wasn't quite elite - merely very good. He wasn't quite a top performer offensively or defensively. Although in the post-season, Fedorov was able to raise it up a notch and help his team win two Stanley Cups (he had 20 points in 20 playoff games in '97 and 20 points in 22 playoff games including 10 goals in '98), thus compensating for this drop-off in his regular season performance.
Peter Forsberg, conversely, was more consistently elite - when not injured - from 1996 to 2004. He racked up 691 points (or 747 adjusted points) in 533 games during this period of time, an average of 1.30 points-per-game or
1.40 adjusted points-per-game!
Additionally, he twice led the NHL in playoff points between 1996 and 2004 despite not even getting as far as the final round.