That's a tough one, but if I were Carlos I'd probably go with Williams. I think James Vowles is on the right track there, and I think he's very capable from the engineering side.
The issue is that they are so far behind the curve at the moment (overweight, archaic systems, etc.), it may be a few more years before that organization is no longer putting out fires and instead making real progress. As good as James can be, he can't pull that team out of the dark ages in one tug.
Having a driver team of Albon and Sainz for the near future could help though.