Although the Boxster isn't for me personally, I would bet it will prove to be better than the M3 in most meaningful ways on a track. It just comes down to what you want/value. The new version is also very nice to look at.
2016 M2 | Long Beach Blue | 6MT... coming
2015 M4 | Austin Yellow | DCT... current
2014 Corvette Stingray | Z51 | Torch Red | 7MT - Sold
2012 MB C63 Coupe | Performance package | Obsidian Black - Sold
2011 E92 M3 | Jerez Black | Fox Red | DCT | Competition Package - Sold