This one's pretty easy to deduce, even though it's a shame to have to diminish one person's accomplishments to elevate another's. Nevertheless ...
If Cal Ripken Jr. had done what he did in the same year Michael Phelps won his eight Olympic gold medals, who would earn the SI Sportsman of the Year honors?
The answer is as simple as the answer to this: Which is bigger, major league baseball or the Olympics?
There is no grander stage than the Olympics, and doing what Phelps did on that stage, against the best in his sport, with the eyes of the world on him, eclipses even Ripken's record. Actually, the debate should probably be limited to other Olympians - Carl Lewis, Eric Heiden, the Dream Team, the U.S. hockey team, for example. But that's a Faceoff for another time.
Baseball wishes it could command a stage a quarter of the size the Olympics do (of course, if it could, the Olympics would not have given the sport the boot). In fact, had baseball not given up its place in the sports universe years ago as carelessly and pointlessly as it had, the Streak might never have had to carry the weight that it did.
Overall, there are other past Sportsmen or -women to whom Phelps might have to take a back seat. With all due respect to the Iron Man, Ripken is not one of them.