How would you rate them in comparison to something like the Swiss K31?
It really depends what you want to do with them. I have one of each, K31, MAS 36, MAS 49 and MAS 49/56. If you are interested in collecting them, they are pretty much the same, by that I mean they are high quality military surplus, manufactured with high quality materials, with a well documented history including rigorous markings and stamps on the rifles when they went through an arsenal for a refurb. You know what you have in your hands; no risk of forgeries, "field mods", "rare variations" popping out of nowhere, etc...
If you want to shoot them, they are all pleasant to shoot. Obviously the actions are different so the shooting experience goes with that. The MAS 36 has an interesting bolt action that is very fast to operate and which I can cycle without breaking cheek weld. The MAS 49 is the heaviest of the four, but so far it is also the most accurate for me. The 49/56 is very handy for a battle rifle of that era. The K31 is also pleasant to shoot but I have to break cheek weld when I operate the bolt unless I want to poke my self in the eye with the safety ring. The sights are about the same for all four, although the MAS system of fixed sight is a real pain to adjust if it needs to be. The MAS 49 and 49/56 have milled railed for a scope mount which is really handy and if you can get your hand son an APX 806 is greatly improves the shooting experience. I understand that you can get an after-marked no gunsmith scope mount for the K31 but I haven't experienced it. Ball ammo is reasonably available for both calibers 7.5x54 and 7.5x55, I find mine from CGN sponsors and at trade shows, and apparently some people have been lucky with their local CT stores of all places. Reloading is also an option, and CCGN sponsors carry all the components that are needed for that.
If you want to hunt with them, neither guns is well adapted to that. Its a common joke that a gun collector is easy to recognise during hunting season because he will be the one carrying on overly heavy rifle with a awkward custom scope mount with antiquated furniture, sub optimal bullet choice, etc... These guns are no exception from that point of view. Also, the MAS 36 does not have a safety, which limits your options when approaching your game. I know of at least one nature of commercially available Soft Point bullet in 7.5x55 but none in 7.5x54, so if you want good hunting ammo you will have to reload.
One significant difference is price. When the K31 hit the milsurp market, they were relatively cheap to buy and thus a very good deal. The MAS rifles certainly aren't cheap to buy, at least not the 49 and 49/56 I have seen recently. However, you are getting something for your money, in my opinion MAS rifles are not your run-of-the mill bargain milsurps, they are a class above. Again my opinion.
I hope this helps.