Yeah, as mpauly said, they both have their place. Metallic is a nice, (very) mildly abrasive polish that just happens to have some wax in it.
If *I* were going for a really perfect, Klassed finish, *I* would even use Metallic before AIO. AIO would presumably strip off whatever wax/silicon/whatever the Metallic left behind and prep the surface for SG. Whenever I use Metallic (after 1Z's Paint Polish), *I* notice an improvement. I had Accumulatorette check out an Audi A6 I was doing and SHE noticed it too (no, I DIDN'T let her know what I was up to, it was a perfect "layman's opinion" situation). That car, with marring I did NOT completely reomove, looked a LOT better after following the Paint Polish with the Metallic. Applying Autoglym's SRP (which DOES clean similar to AIO) afterwards did NOT lessen the Metallic's improvement.
So far, I've used Metallic under Autoglym's Super Resin Polish and P UPP. No bonding issues with either of those two products.