Blue 253 with M20and 3:08 will smoke a blue 253 with M20 and 3:36. The reason is you can hold it longer between gear changes. There is bugger all difference in take off pick up. I would not use a M21 with a 253 unless you like slipping clutches.
However if you have steep tight driveways 3:36 probably won't be low enough.
But to be honest, the best diff is the one you don't have to pay money for. And at the end of the day most new cars are quicker than a 253 around town these days anyway.