Well, since bogg started it, I have a video of the M coupe vs the Cayman S.
I know it's not a boxster but maybe it will help?
Remember also that the M roadster is quite a bit cheaper than the boxster S.
http://www.edmunds.com/insideline/do/Drives/Comparos/articleId=117041
I'd say the boxster is BETTER, but for the price, the M is a better deal, because it's faster and performs nearly as well.
So if money isn't an issue...Boxster S all the way!