I would like some advice on what we should run from our basement up through the roof for immediate and future condensing water heater venting requirements.

While I have not yet decided on which water heater to purchase, I am at a point in the remodeling project where I need to run the venting.

Condensing tank heaters such as the AO Smith Vertex uses two 3" PVC
Rheem tankless uses two 3" PVC
Rinnai and other tankless units use a concentric system.

We can run 2 3" pipes up into the attic and then go to a concentric terminator, or run a concentric pipe all the way up.

Which would give me the best long term flexibility?

Are there adapters that split a concentric output into separate pipes?

Thank you for your help!