It's a subjective question only you can answer at the end of the day. I own a 10" LB, and if i had a choice between a 10 + nagler or 12+ plossl i'd go the 10" as looking through a nagler will provide a much more pleasing view then through a plossl if your interest is nebulae and globs...
Ofcourse aperture fever will kick in then and you'll say "If only I bought the 12"..."
Other option is buy the 12", enjoy for now with the plossls then upgrade your eyepieces later... that's my 2cents.