PVC vent pipes or concentric vent pipes for tankless gas water heater?
Concentric PVC is a thing. I like concentric pipes because they only require a single penetration rather than two. Honestly, any of your options should be fine for a high-efficiency gas appliance and a three foot pipe run. The exhaust temperature of condensing, high-efficiency gas appliances is pretty low, which is why PVC is an option. PVC might be expected to have greater longevity since it can't corrode, while stainless steel can (it's stainLESS steel not STAINless). And it should be cheaper in materials and labor too. Your gas plumber probably prefers it for those reasons, whether he passes the savings onto you or pockets the profit.