Depends on the year model of your car.
From about 1980 to 1986, the 3.73 diff went with the M46, but from 1986 the manuals have the 3.31 diff.
As to which is better, it depends on the engine, tyre size and what you are doing with the car.
I like the 3.31 diffs for open road driving, but find it annoying around town - suburban corners that you expect to drive around in 3rd gear need second.
But they purr on the open road, and the fuel consumption is better.
