SMB Market: Server Operating Systems

Like it or not, Microsoft is the big dog and Linux is the low cost alternative solution.  I believe that 90% of the server population in the SMB market are probably Windows or Linux based.  At the same time there are a number of other server types in the SMB market as well.  IBM I-Series (AS/400s) platforms, a number of UNIX variants (HP-UX, AIX, SUN, etc.).  I am looking to get a polling from our web audience.  If we were to investigate a third or fourth solution set - where should we invest our time and money?