The SX610 has plastic arms covered by thin metal. I have a bunch of them and the only problem I've ever encountered with them is broken pulleys. There doesn't seem to be any play in the plastic arms. The SLJ6000 has similar geometry, but is all metal with an aluminum cage and pulley bolts. It's lighter and prettier, but I don't think it shifts any better than my SX610s. I use Retrofriction shifters on two of my Simplex-equipped bikes. The other has Criterium shifters because Retrofrictions won't fit. They're metal shifters, but they have the plastic wing nuts that always break. I keep some spares.