The argument for Bourque over Leetch defensively would be some combination of:
- The eye test. I feel confident saying that most objective fans (ie excluding Bruins and Rangers fans, who could be biased) who watched both of them play would rank Bourque ahead defensively. I actually think that Leetch was underrated defensively, but I see a clear gap between them.
- Polls of peers. Bourque was named the best defensive defenseman in the NHL in a January 1994 survey of coaches. He finished 2nd in the February 1993 survey of coaches. And he was 5th in a January 1990 poll of players. Leetch never got votes for best defensive defenseman in these (or similar) polls.
- Plus/minus. This is a very limited stat, and it tells you at least as much about offense as it does defense. With that disclaimer -Bourque was +157 in the 1990's (first on the Bruins by a wide margin) and Leetch was +47 in the 1990's (4th on the Rangers). Again, this would be the least persuasive argument out of these four.
- Goals against. During the 1990's Bourque was on the ice for 686 non-PK goals against in 735 games, while Leetch was on the ice for 740 goals against in 722 (that's 0.93 per game for Bourque vs 1.02 per game for Leetch). During that decade, the Rangers only allowed slightly more goals than the Bruins (2,377 vs 2,362 - a difference of less than 1%), so I don't think the strength of their teams made a significant difference. I'm doubtful there was a significant difference in their ice time to explain this difference and I suspect that, if anything, Bourque's matchups were slightly tougher.
I think it's important to consider defensive play, but it only increases the gap between them.