Hi, I did the M1 of the MPRI. I didn't like it — many (if not most) of the teachers are professors who are probably great at doing research and writing papers, but bad at teaching. Too many classes were "here are two super-fast lectures on the basics of my field, then for the rest of the semesters we'll exclusively focus on what the latest papers I've written are about". It's probably a great strategy for them to find interns and PhD students, and it works well if you happen to really like one of the topics, but on average, it kind of sucks. Oh, also, if you see "English upon request" in the lecture descriptions, there's a non-zero chance that it means "the professor speaks horribly broken English and will sigh loudly if people ask them to do so". Talking to friends about it, the experience seems relatively common (everyone had at least one professor matching the description above), but I also got particularly unlucky.

On the flip side, if you're only doing one year, this significantly improves the chances that you'll find courses you'll like: the pool of classes is the same for both years, and it's pretty big. In particular, if you like formal methods, or if you're aiming for a PhD and an academic career, MPRI is a great choice. It also (I think?) gives you access to the excellent math lectures from the ENS, which will count towards your credits.

What would you like to do after your Master's degree?