Mate I am a 5000 hour plus line training Captain and also 1000 hour Instructor at PPL level. The flight simulation engine is a pile of rubbish.
Maybe I have a little more exposure to instrument flying (3000 hours manual flying in a old heap of a TP) and teaching than you have hence I think RANT is pretty good and worth 80 quid and MSFX a pile of poo for most instruction outside having a wazz around to get the flow of a procedure with radio setting nav aids and stepdowns, turns and speed reductions. But if the basic skills arn't there to begin with or they are just maturing it can do more harm than good.
And I get enough flying daily so as such I don't have a copy of either. Skyrim gets a good hammering though.
RANT is not a simulator it is a radio navigation tutor. It is a stuctured course to understand the instruments, the information they are giving you and how to interprete it.
I'm currently doing the ATPL(A) theory, but want to get ahead of the game in my spare time.
As this is what the OP wants. Rant fits the bill exactly. MSFX would just be a game with potential to give issues when real instrument training starts.
If during the proper instrument training they think that MSFX may be of help they can spend a fiver and crack on and use it in a strutured manner.
But the FO's I line train usually start off with that theory but quite quickly stop using it because it just ain't an aircraft doing 210knts with ATC blaring in your ear and some moaning line training Captain in the LHS seat telling you to keep the speed up. Usually because you have a heavy up your arse and ATC will blow a fuse if you come back to 140knts at 12 miles or 160 knts on the downwind.
So may be thats the reason why we will have to agree to disagree well until you get a load of teaching hours and operation hours under your belt then you might agree with me.