I think, the lap difference isnt so big that this must be discussed.
Itīs a question of your envroiment, your preferences and of course of your purse.
M3 is car with a excellent price/benefit-quote and ambitions for also the track and the daily driving.
997 is a track car which can be used on the normal road very well, itīs expansive and brings a very good reputation with it.
I think its like comparing apples and bananas.