A lot depends where you are.
I've got both, C2's on a 32h Powertap , which replaced a worn out Open Pro, and had loads of OP's built on various hubs, either for myself, or I've built for others.
Here in the UK, the C2's are damn nigh impossible to get hold of, and when you can, they're nearly twice the price of the Open Pro's. If you're going to be riding in crappy conditions , thats got to be a consideration if they're not going to last...
Mavics = readily available.
Weight wise, they are about the same. From memory they were about 435g each.
Build quality;I've only ever seen 1 of these masses of open pro's with "rattley" bits or eyelets ripping out. I'm sure there is poorer quality of both parts floating around out there, but by virtue of the mavics being a far greater seller, there are bound to be more of them.
Spoke count? Well its not too hard to get an OP in 28h, but the C2's although offered in various drillings, seemed impossible to track down..
The OP is easier to get a turbo tyre on too
Purely subjective stuff but- i'm sure i've seen a few posts on here about clearance with wider rims on various frames; might be worth checking first...
and also (as he ducks for cover) im not exactly sold on this wider is better philosophy. I couldnt really tell any difference in performance.
Cant comment on your choice of hubs, but the Alchemy's are rated highly on here.
I guess which spokes you use will be determined by which hubs you use, and spoke count and build pattern.
Happy not to have cleared anything up!!