Tough question...
I'd find out first what condition it is in and whether or not it is a 308 or a 308B.
$150 is a good price for a healthy 308B. I paid $50 for a trashed 308B.
A lot of similarity in functionality between the M-30 and the M-308. They are both cool-cool mixers, but my hands-on experience is with the 308 and so I think its cool-cool-cool. Lots of features packed into that frame...the 308 is surpsingly full of 5532 opamps which are arguably a step up from the more commonly used 072, and I believe the M-30 uses 386's and 4558's, BUT...(blahblahblah)
Here's the big question: are you at all unhappy with the M-30? Remember, its cool-cool...I'd say stay with what you have unless the 308 has a feature/features that you need or want that is/are lacking on the M-30, or if you know you like how the 308 sounds better than the M-30. Make sense?
My 2p.