If you are going to be post-producing on a typical DAW, it doesn't matter if you de-matrix the MS signal to XY in the recorder. There are plenty of free plugins which will put it back to MS, then vary the width, and re-dematrix it - eg Voxengo MSED. For that matter, you can use a any stereo mic, (not MS), then convert to MS, play with the width, then dematrix, in your DAW using such a plugin.
In theory - some say - there's no theoretical difference between a dematrixed MS recording and te equivalent XY. Personally, I think there is a difference due to having the centre of the stereo image right on axis of the "M" mic - but you could spend a lot of time kicking the math of that around to prove or disprove it, which I'm not about to do!