sound quality between ARC Ref 150 and mono 250

I am debating between ARC Ref 150 (stereo) and Ref 250 (mono)

The 150 is more in my price range, but if Ref 250 is significantly better (sound, imaging, stage, etc), then I might consider spending the extra money.

Right now I have Magnepan IIIa, but am thinking of upgrading to 3.7, or even 20.7.

Online research seems to show that people have been happy driving the Maggie 3.7 with just the Ref 150.

Both 3.7 and 20.7 are rated same sensitivity I think, so is it fair to say that Ref 150 could be sufficient even for the 20.7?

I've never owned mono amps, but everyone claims that mono offers better separation, imaging and sound stage. But if Ref 150 is such a great amp, will the sound quality difference between a great stereo amp and a set of great mono amps be very obvious?

thanks in advance for your inputs