My wife and I just finished up 2 nights at each of these properties last week. For sure, the Regency was our preferred property, but we're also both 24. It was much more fast paced and lively; the pool was a lot more active, and everything around was much more reasonably priced. If you are looking for just relaxation and an "upscale" feel, the Andaz is the way to go. That is not so much our style, though. We found ourselves a bit bored at the Andaz.
We were upgraded to a full ocean view in the middle tower at the Regency, being only Platinum members, but received nothing (not even recognition of status) at the Andaz. , even though it wasn't necessarily expected.
When we return back to Maui in the future, it will be a full Regency stay, no doubt about it.