Hi!
I noticed you haven't got a reply yet and only have a couple of days if you haven't decided, so though i'd try and say a bit if you still need the help.
I'm taking M140 similarly to you but was only allowed 1 module to begin with because of my age and circumstances so have to choose my feb modules. I've been advised not to take MST121 as MST124 (first available in feb) supposedly leaves near to no gap for other modules whereas 121 does. MS221 isn't being replaced for another year as far as i'm aware so i'd go for the one maybe?
As for the difficulty, i took statistics at A level for maths and further maths... I've been told it escalates to just harder than that so i'd assume closer to MST121, but the advice might be wrong, but hoping not :s
Good luck! x