From the current density or current-per-via viewpoint - either design A or B may be better, depending on the pattern of the current flow.
For example, if the current flowing from M2 to M1 is split equally into two branches - then design A is better, as each via will be carrying one half of the total current in design A, while in design B the current in the upper (in Y direction or X-Y coordinate system of the GDS layout) via will be mush larger than in the lower via.
If the whole current in M1 is flowing up (in Y direction) - then design A and B are equally good (or bad) - as the whole current will be flowing through one (upper) via.
If the whole current in M1 is flowing down (in Y direction), then design B is better as the current will be distributed between the two vias, while in design A it will be flowing through one (lower) via.
It's very instructive to run a few simulations for current flow in interconnects like this - to quickly gain a physical insight into the current flow patterns, and get a better intuition. Then next time you do not need even to run any simulations - you will know what the optimal layout is, depending on the expected patterns of current flow.