Is there any advantage to buying a 6.5 glass carboy vs a 6 gal carboy? I’m looking to make the upgrade from plastic to glass.
If you are brewing exactly 5 gallons, a 6 gallon should be fine. Some people like to brew 5.5 gallons to compensate for things like dry hop absorption and trub loss so they are putting 5 gallons into the bottling bucket or keg. If you are brewing 5.5 gallons, you will result is an excessive amount of blowoff with a 6 gallon carboy, so the 6.5 gallon offers more head space.
The 6 gallon ones are used mostly for wine.
I use a 6.5 gal one for all of my 5 gal batches. In my opinion, it’s the ideal size (except for the 7 gal one I used for many years until I accidentally cracked it).
I like not having to bother with a blowoff tube and I like not losing any volume thanks to the extra space…
They do tend to be slightly more fragile than the 5 gal ones though and of course they are heavier than plastic. I still have a bunch of 5 gal ones around that I use for secondary (a step which I stubbornly cling to) and for bulk aging of certain brews.
You should look at those plastic carboys. I got a couple of the Bubblers from NB and really like them.