I would buy both, I4 or I6, however due to emission standards, fuel economy, downsizing and other regulations, i think it will be a lightweight/I4 combo.
The legendary E30 M3 was 4cyl, lightweight, superb handeling, it offered specially engineered parts like gearbox, chassis, suspension, diff etc... E30 6 cylinder models where available at the same time...
M2 like all other M/// cars should offer a package with specially engineerd M parts like lightweight bodyparts, (carbon roof), bigger (better) brakes, suspension, chassis, wider track, wheels, exhaust, interior/exterior details (seats, wider flares, fenders...).
So one can sepperate a true M/// car from the other "normal car" perhaps upgraded with performance package like M235i.
For the matter of the 4cyl, let's take a quick look at the 328i engine.
Simply adding the hartge software brings it up to almost 300hp...
Imagine the bmw engineers giving it the "alpina-treatment", upgrading the cranckshaft, pistons, inlet manifold, exhaust system, extra (better) oilcooling, extra watercooling, sparkplugs, cam's, mapping, using magnesium/aluminium alloy cast for enginebody, higher redline
I'm guessing it could easilly put out 300 plus horsepower.
For the matter of sound of 4cyl. WRC cars (turbocharged) today make a great sound...
Personnally E30 M3, 2.5l with carbon airbox, OMG what a sound! When in front of the car, only induction noise. Good times! When in back, only exhaust...
Sure, there is nothing like a N/A I6 like the s54, but as previously mentionned, due to emmissions etc, we've probabbly seen the last N/A engine in the E90/92 M3. It 's truly sad, but i'm guessing we will never see N/A engine in a BMW road car in the future again.
I'm changing cars in a couple of years (2), but before i decide, i'm looking at the new M3/M4, M235i, and of course M2. Waithing on the reviews of the journo's befor i buy one of these cars. I would consider the alpina B3 biturbo aswell or M135i.
(sorry for my bad english, i'm dutch/French speaking belgian)