my opinion:
the quicker way is to do a new clean install with 6.x.
if you have many contribs that are not compatible with 6.x, you have much work to remove them and install the new versions.
i read here that some people had problems with the upgrade, others had nothing.
i did the upgrade last week from 5.6 to 6.0.1 by a fresh install on a new harddisk. installed the needed contribs and got a clean SME.
cheers