I have a convertible as my daily driver so my opinion is based upon comparing the convertible to a 911. If this is just a weekend car, I'd go for the 911. I've had a 997S and a 997 turbo and currently have 997.2 GT3. I'd take any of them over the M3 convertible if just a weekend car. As a daily driver, any M3 really hard to beat. It is just so utilitarian compared to a 911. However, if push came to shove and I could only have 1 car, i'd still go with any 911 over my M3 convertible. To me, its a different animal. Just more raw, engaged and entertaining. But I'm very fortunate to have both and thank God and my wife every day.
2008 BMW M3 Conv
2010 Porsche GT3
2007 Porsche 911 Turbo (gone)
2007 Porsche 911S (gone but always remembered - 1st Porsche)