.4 None of the above.
No offense, all good cars, but represent the least desirable of their series.
Personally, I'm somewhat biased against the M3 convertibles- (weight, safety and handling compromises -not to mention the "poseur" factor) as it seems contradictory to the M3's original purpose. Yes, I know people buy them, however it has been well stated that the "M" is now more representative of marketing rather than motorsport. Again, I admit my bias as an E30 M3 traditionalist. (No flames please.)
Less subjectively, also note the that the convertibles resale value is directly correlated to this lessened desirability as evidenced by their lower prices than similarly equipped coupes. Here in the Sunshine State where 'verts sold new aplenty they seemingly fetch less used than coupes of the same ilk despite the original price premium.I adore my E46 M3 coupe and most enthusiasts appreciate the additional rigidity and onelessthingtogowrong reality factor of buying a preowned German car presumably out of warranty.
It's puzzling that you would simultaneously compare a rear wheel drive convertible with a newer but comparatively underpowered four wheel drive coupe. If you're in the snow belt and need the traction, I think the choice is clear. The E92 coupe, comparatively speaking, will feel like its carved out of granite from an overall rigidity standpoint. However, it is also the smallest engine in the U.S. range with the additional weight penalty of four wheel drive mechanicals. Dynamically, it would be far less satisfying than the aforementioned E46 M3 convertible having far less power, four wheel drive understeer and increased ride height.
The solution? I suggest you opt for the better of the breeds. Narrow your selection to e46 m3 coupes and perhaps preowned e90 330i sedans which are at comparable pricepoints. Both offer a performance advantage over their series counterparts. From there, it becomes a little more subjective based on your tastes and needs. The economic climate of the market affords such a plethora of choices that one has the luxury of being both patient and picky. Don't limit yourself to just three choices!
To me, its M3 coupe vs 330i (or 335i in same price range) again unless you REALLY need four wheel traction.
I like the e90/92 cars, but I'd go for the E46 M3.
But that's just my two cents!