I really don't think it's that debatable. Potential health and the difference in contracts are the reason they got picks out of the swap, not because they're equivalent players. There's a reason he's a perennial max-ish guy and Smart will never sniff it.

Porzingis is an efficient paint scorer, especially for your third option and with Rob as pure vertical threat. He's not just a catch and shoot guy on offense. He might not be your best scorer, but he's a damn good third option, and he's a great option against a zone team like Miami that wants to push you into taking short-mid range shots without getting to the rim.

Smart is a good defender, but he was far from a special one last year. He wasn't better at the point of attack or on the perimeter than White, and his standout trait defensively last year was that he wasn't a mismatch against bigs. Porzingis isn't a great defender, but he's also not a liability, and swapping him for Smart means you're even bigger while still not having anyone on the court that's easy to blow by. It also means Tatum can play 3 and doesn't have to battle in the paint as much to allow you to get rebounds.

Brad told you they had surplus at guard because they did. They want to play bigger and this lets them do it.