I'm assuming your asking this question in present time.

JMM has never weighed in at 147 so I really couldn't tell you.
If I had to pick a winner I'd say SSM.

At 140 I'd favor JMM.
For starters I think the weight drop would effect SSM in a negative way.
Similar to when ODLH dropped to 147 to fight Pac just not natural and it would be too much. SSM hasn't weighed in at 140 in 11 years.
On top of that because I think SSM has completely lost it now.
His hands and feet just don't respond anymore.
I think with the right opponent he can score a win but as far as beating a current top 10 p4p fighter within his range of weight I don't see that happening.

Possible opponents whom I think SSM can beat.
Mike Jones
Luis Abregu
Isaac Hlatshwayo
Cornelius Bundrage
JCC Jr.
Carlos Quintana
Joel Julio