I had aluminum in the Ion/Freak Jr. No problems. Norm is using the aluminum inserts in his A5 and Sp1. They are working well.

I have no experience with the SS inserts, but SS will hold its shape better and wont scratch as easily, and wont change as much with temp differences. I really dont think theres much of a difference in performance, really only durability. If you take care of your stuff go aluminum. If you let it get dirty and have the inserts rattling around at the bottom of the gun bag then your best bet is stainless steel.

^What Chris said. Also IIRC SS doesn't have a .679 insert so you'd have to pick that up in aluminum separately.