If you have a choice of both I would have said down as the synthetic belay jackets I find are better for our damp winters. Hydro down could be an option for you. March is still a bit early for the warmest weather so I'd pack warm for the nights.
I've been up there in early March. I had thin thermals, skinny down jacket and an alpine start jacket (basically an extremely thin windproof layer) on; that was fine. Drying things out could be a pain so synthetic might have its advantages.
Judging by the facebook photos I was in a windcheater (alpine start) and thermal most of the day, from memory the nights got cold, so ensure you've got stuff that you can be comfortable in when not moving.