A brief comment about my own comment
With the MB500 pushing it, the NY121 will probably be significantly louder than the MB150.
This is not necessarily a true statement. If the efficiency of the 12" speaker in the MB150 and the NY121 is the same, it's likely you could get more volume out of the MB500 with 350 watts available pushing the NY vs. the MB150 by itself (75 watts) or with the 112MBX (150 watts).
If the MB150 speaker(s) was significantly more efficient than the NY, the volume thing might be closer to equal.
Another thing to consider, the NY is ported and the MB150 & it's extension cab are both sealed. That will probably make more difference in your perception of the sound out of each.
Bottom line to me would be the MB500 and the NY likely gives you more juice & still has the portability. If you do happen to get in a situation where you have an 810 waiting on you when you arrive at a gig, and you need it, the MB500 in your gig bag will really be handy.