Everybody is talking about #1 offense vs. #1 defense, and almost everybody picks offense (Lakers). But what everybody forgets about, is that the difference between the Lakers' defense and C's offense is much bigger, which should favor the C's.
Actually, your statement is only true for the regular season, but not the playoff:
In the regular season, Celtics score 100.5 and opponent score 90.3. 100.5-90.3 = 10.2 points different.
In the playoff, Celtics score 91.6 and opponent score 87.3. 91.6-87.3 = 4.3 points different.
On the other hand, Lakers:
In the regular season, Lakers score 108.6 and opponent score 101.3. 108.6-101.3 = 7.3 points different.
In the playoff, Lakers score 105.9 and opponent score 99.5. 105.9-99.5 = 6.4 points different.
So stat wise, Celtics is a better team in the regular season and the Lakers is a better team in the playoff.