You could strip 150kg of weight out of your M3 that might make it more interesting
As others have said both very different cars,
I've often thought about an E46 M3 and it would be safe to say I have a soft spot for the Bavarian mark but the "wet track day UK" means blasting round in the wet and humiliating lots of far more expensive equipment would no longer be a reality with an E46. This stops me thinking about it any further, that coupled with the expensive running costs you mention.
As for it being more of a drivers car.. I would say it all depends how you have your Subaru or M3 setup. Out of the box I would expect the M3 is a bit better and more refined but fettled with some decent mods is a different story. All said and told pound for pound you will be travelling a lot quicker in the scoob for your money than you will in an M3 for the same budget.