Cannot find a 600 anywhere now
Just follow bandphand's advice.
The most important thing between the 151 and 600M is the inherent ControlCAL ISFccc stuff, so that you can have multiple calibrations for
each source. Think night vs day.
There is an unauthorized patch out there for the 600M, but I believe it voids your warranty. Now, this doesn't bother some people whatsoever, whereas for others it's a big deal when you're spending this much.
Last I heard, the 600M pricing was raised from the really excellent deals available for a bit there. I suspected the end of the stock was near, and I guess it must be over now.
In any case, that $500 extra for the onboard ISFccc could be worth it to some people.
That's still a good deal, IMO. Best Buy Magnolia is selling the 151 for $7,000.