Chalk and cheese.
They are completely different boats. Only similarity is that they both have pointy ends and blunt ends.
and even those bits are different.
Monsun is a full keeler (designed by Olle Enderlein) and the 31MkI (German Frers) is fin.
Have a look at the sail plan of the 31 and the brochure of the Monsun. You'll see the differences straightaway. Interior drawings also show major differences. Extra beam in the 31MkI, galley, head location.
I'm sure the Monsun is a fine boat but the 31 has got to be a better all rounder if only in terms of comfort and speed.
Mind you, I betcha prices vary a hell of a lot as well.
All in all though, and admitting that we own a Swedish boat (Malo) so a certain amount of bias, I'd happily recommend them to anyone. Vindo, Najad, Malo, HR .... fine boats all. Previous owner of our girl has now got into an HR 46 and he reckons the Malo is slightly better build quality. I'd still be content with any of them though you won't be taking home any club race trophies.