Are fixed-weight or adjustable vests better?

It depends on use. Reviewers favor fixed-weight neoprene vests (like ZELUS) for simple, low-cost cardio and walking, while adjustable block-based vests (like Eric Flag, MVRK, or the CAP 20–150 lb) are preferred by calisthenics and strength communities because you can scale load over time. The tradeoff cited across reviews is cost and bulk.