Should I choose the 1/3 Century or Metric Century for my first organized, supported group ride? In one week I will participate in my first organized ride since buying my road bike in May. The longest training I've done is 28 mi averaging 14-15 miles/hour on chip sealed roads with maximum elevation gain of 800 feet. Both routes will be similar terrain and elevation change.
You're asking should you try to ride 33 miles or 65 miles when your longest training ride is shorter than either?
I think you'd be much better off trying a 60 mile training ride before the event. Sure, you could probably do that on the day, but you could easily end up either failing or struggling through and feeling miserable.
If you think the "supported" part will make a difference perhaps try carrying a bit of cash and choosing a ride that lets you stop a couple of times at shops where you can buy snacks and drinks. Personally, I'd carry both on the ride. A extra couple of pounds isn't going to make any difference unless you're right on the line before complete exhaustion stops you. And you shouldn't be there.
It depends how comfortable you were feeling at the end of the 28 mile ride.
My rule of thumb when advising people is that if they are quite comfortable riding one distance, then they can probably ride for half an hour more at a similar pace, or 20% further if they keep to a lower pace overall.
In your case, the answer comes out the same: do the 33 mile ride.
60-odd miles is a long, hard ride if you are not used to such distances. I would only recommend it if you were riding 50 miles comfortably.
Supported rides are a good situation for extending your limits, but I recommend that you take advantage of the support, rather than rely or depend upon it.