1. If the contractor is not proficient in gravity systems do not even consider it since it has to be "engineered" to very strict guidelines, and ALL the plumbing has to be higher than the water heater.
2. Forget about ever recouping the cost of a circulation system. The added fuel costs, electricity to run the pump, and shortened heater life will add to the long term costs, not reduce them.
3. Those systems are not a panacea. They give "warm" water initially, but still have some lag for "hot" water, and the cold water faucet may have an initial spurt of warm water.