Big fans of both, and I think they have a pretty similar value right now, which would make a fair trade. That said, I probably wouldn't do it, mostly because Smart is my binkie, though Noel is my non-Celtic binkie.
However, if Smart doesn't show improvement this year offensively as the leader/ball-handler of the second unit, then I'd probably pull the trigger at the deadline. Smart definitely has a much higher two-way potential than Noel, but if he gets the opportunity to succeed this year and still doesn't show much improvement, then he'll probably never be more than a Tony Allen-like high level defensive role player. Noel's potential to anchor a league-leading defense in the middle is probably more valuable to us long term than Smart's perimeter D, which is definitely special but expendable with Rozier, Brown, Bradley, and Crowder.
Best case scenario is for both Smart and Rozier to make a leap this year, and we use Rozier and the Memphis pick as the trade bait for Noel.