Agreed, a 1200W Platinum modular PSU will be more than enough for your build, even if you overclock your i7/GPUs/DDR4. You'd really only need 1500W if you plan to pack additional GPU card(s) or a lot of drives. The RVE User Manual recommends a minimum 1000W PSU for multi-GPU configuration.
An under-loaded PSU will generally run cooler and quieter (less electrical strain, less fan activity, etc) than a fully-loaded PSU, so I would expect an AX1500i to have greater overall longevity and suffer less electrical "aging" (which de-rates performance) than the AX1200i on your ~900W-loaded system. But the best assurance that a PSU will last you a decade is to buy one which comes with a 10-year warranty, like an EVGA SuperNova or a Rosewill Photon - or to buy a server-grade PSU from LEPA, FSP/Zalman, Seasonic, etc. I personally think you can never have "too much" PSU! But I wouldn't pay extra for the 1500W product unless I really need the power/features (or plan future upgrades which really need the power/features) or unless it happens to be on sale at a fantastic price. Better to spend budgetary limits on upgrading other components.
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