I got to handle both polishes at DetailFest at the Rupes booth on a hood atop a stand.
Having come from a PC, GG6, and 3401, my initial impressions are as follows:
1. the LHR15ES is smoother than the 12E. Noticeably less vibrations. Also easier to keep steady than the 12E.
2. the 15ES sounded quieter. Or maybe it just sounded smoother. Hard to determine with everyone around you under the vendor tent.
3. The 12E Duetto felt more powerful. It was harder to bog down when applying downward pressure than the 15. Alot of people like this characteristic, basically a PC on steroids. If you're comfortable with a PC/GG6 then you're most likely going to like the Duetto alot.
4. The Duetto sounded noisier, was less smooth than the 15 but way smoother imo than the GG6 or PC, vibration wise. It tended to walk a little bit when trying to hold it one-handed, whereas the 15 did not and was rock solid.
I am torn between the two, I keep going back and forth. I intend to pair whatever I get with a 75 Mini for the hard to reach areas.