I do the same as in Sacred 1:
level to 35-40, check if you enough runes (not so much of a problem in Sacred 2) then hit silver and do EVERYTHING ( = exploring, dancing, drinking, side quest,...),
then rush Gold, then do EVERYTHINg in Plat, then try to survive Niob if not get back to Plat.
In MP I just grind my way in Silver then when I got the right level move on to Gold etc...
Nowadays I stick to MP, will probably play SP through on Bronze for the Storylines of individidual Characters.
cheers!
Chareos Rantras