In my personal opinion, I think Button / Alonso would be the stronger pairing. Button may not shine when the car is fighting among the midfield, but when the car is right and fast (which it just might be), I think he could be quite a contender to Alonso, perhaps similar to how he managed to impress alongside Lewis through smart driving. If anything, he is a very consistent performer, which I think is what the team needs.
Magnussen has not impressed me honestly, but then again; it's always hard to judge two drivers when a car is not performing well. Which is why I think Perez got shafted a bit unfairly out of McLaren a bit quick last year as well, and perhaps why Magnussen may be this year, if he doesn't get to stay on.