Should I use 2x16GB or 4x8GB RAM for stability?

2x16GB is the safer bet for stability. Two sticks are easier on the memory controller than four, with less electrical load and more room for XMP/EXPO to run at rated speeds without crashing. Four sticks often force the memory controller to drop speed or timings, and you might end up at 3000 MHz instead of 3600 just to keep the system stable. If you need 64GB later, start with 2x16GB and add a second kit then.