MPA chassis is the most popular and allows for a great deal of customization. Manners stock does not. If you were to take a shooting fundamentals class, or do a lot of competitive shooting where you might want to raise/lower cheek, or adjust the buttpad from Prone to Offhand shooting in the middle of a CoF, MPA would be the winner.
If you like the looks of Manners, which looks great... then that's that.
If you were trying to find something light to walk around the fields and kill squirrels, then Manners would also fit the bill. I wouldn't buy a competition chassis, and haul that extra 3-4 lbs of weight unless I meant to do some competitions.
I have a KRG Bravo right now in my setup for my Tikka. It's a great all around stock, light enough to carry it, stable enough for multitude of shooting, but I am looking to buy something like the MPA or MDT for more customization for competition shooting.