Materials that splinter rather than crumple would be problematic, but assuming we keep cars constructed basically as they are now, I would expect the collision between lighter cars to be better for basic F=MA reasons.

ETA: I stand corrected: See here on page 4:

The results have a clear pattern: reducing a vehicle's weight increases net risk in collisions with substantially larger and stronger entities, reduces net risk in collisions with much smaller and more vulnerable entities, and has little effect on net fatalities in collisions with vehicles of about the same size (although nonfatal injuries increase).