You cant go wrong with the standard steel ring. Generally speaking the small ones are small and the large ones are pretty large. I personally use an aluminum ring but after the Kong ring failed a lot of people were turned off aluminum rings.
The ISC rings are proof tested to 16kn and rated beyond 25kn, thats the one I use.
https://www.treestuff.com/store/catalog.asp?category_id=303&item=880
I also have a swivel on my bride, which I put on, then took off when still climbing DRT. I am giving it another go to see how it works with SRT dynamics. It was a hassle DRT. Some people love it, others hate it.
I do like having 2 connections, either two rings or a swivel and a ring. I will clip one climbing system in to one and having another is nice for trying a quick closed DRT system as a second tie-in.
Kevin Bingham uses a rigging plate, If I was going to go that way, I would make sure to use one of the fancier, rounded ones with rope friendly curves. I havent tried that method, but do own some of the CT rigging plates pictured above and they are a good quality hardware with a rope friendly surface. The ISC ring is big enough to clip the HH's Oxan and a Petzl William in behind the HH, a rigging plate might be nice for using the HH to DRT but I cant imagine it would be that much better..
I agree though, I would never use that biner>biner connection again. You could easily shock load a biner cross gate and possibly snap it, leading to a fall.
Being used to a biner. you might like a larger ring since it extends the system out a bit, I personally like a small ring for the opposit, it keeps everything closer. Avoid the ones listed as rigging rings, those are really big.