Both have exactly the same weight.
MX-S is slower, softer and spinier. Better for serves, on the table game and opening against backspin.
MX-P 50 deg is harder and faster, good further away from the table, not so good close to the table. Very easy to finish the point when the ball is high. Very unforgiving too if you don't hit at right angle and speed. On pushes sometimes the ball pops higher than you want, but I guess this is related to the fact I'm using MX-S (softer) on the other side.
MX-S has more grip than MX-P.
I tested both on Nittaku Violoncelo, first MX-P on FH, but I've made too many unforced errors and now I'm using it on BH, MX-S is good on FH I wish it'd be slower with more grip.
When I glued the rubbers on blade I noticed that MX-P curled nicely because of the speed glue. I'll try to boost it because I have a feeling the sponge on MX-P 50 deg was made to be boosted.
I prefer MX-S, but that doesn't mean is "better" than MX-P.
I'm an amateur so take my opinion with a grain of salt. If you're a good player (good footwork, hit at the right angle) you might like MX-P 50 deg.