My favorite is the original metal Power Rings. I also have the Elite Rings which are plastic, and do not like them quite as much. My understanding is that the Gill rings are also plastic, but I don't own a pair.
I like the heavy metal rings because of the heft they provide. Also, my hands stick easier to the bare metal than to the bare plastic. I haven't tried putting grip tape on the plastic, but that would probably improve the grip.
Sorry that I can't provide a better opinion.