I have a few square feet of polycarbonate lying around. Haven't had the time to use it for a greenhouse yet (simply because I still have none).
But what I can see as advantage:
2. Easier to work with, to cut etc
3. Cheaper (maybe?)
I don't think it would hold heat better. For sure it lets less light in, and for sure it loses some of its transparency over time (seen few several years old panels).
So my take is, if your glass is still staying solid, don't change it. But I would try polycarbonate for a new greenhouse.
P.S. I just arrived to the forum, looks like a great place!