@learning to live,
the spec sheet on jemsite lists most features of either guitar, but some details are missing.
Firstly, when picking up the HAM after the Metal you will notice some differences in appearance and features.
1 The finish on the HAM, besides being different in color, HAM= icy blue, metal = matt-silver, with some dark micro spots, it is also rougher. Perhaps the sandblasting on the JEM90 was just a bit quicker. If you plan on playing, while wearing an expensive suit or a woolly jumper (Weddings, X-mass, whatever) the metal has a definite advantage.
2 The back cavity covers on the HAM are matched in finish to the body, the RG-metal has simple black cavity covers. Both are sandblasted from the same template, but the templates are offset slightly, so the patterns don't match from one guitar to the next.
3 The RG-metal has metal domed knobs, the HAM uses black knobs with white numbers. The RG-metal should have a silver tipped selector switch, often this is missing and replaced by a simple black tip.
Secondly, the neck is different.
1 The JEM90HAM is flat-wide, whereas the RG-metal is more vintage D, it actually feels narrower. The RG's frets feel smaller. I'm not sure if the official dimensions reflect this.
2 The Rg-metal has a full complement of offside dots, in contrast to the HAM which has only a 12th position double dot and a 24th fret anniversary plaque.
3 The RG-metal neck has a volute
4 The RG-metal neck is three-piece maple, the JEM90 is maple-bubinga-maple.
5 The headstock on the JEM90 matches the body.On the RG-metal it is black.
6 The RG-metal has a (metal) trussrod cover
The typical JEM vs. RG details are obviously there, monkey grip, angled jack, lions claw.
Lastly the pickups will make a huge difference in tone, but that is nothing compared to the difference in feel.
@akoch
simply stating that it's a J-custom doesn't mean a thing, as the workman ship on the anniversaries is way better than standard on MIJ signatures. The little details (cavity covers, headstock) on the HAM vs. the metal show that the 90s are a small step above the metal. In fact I would say that the JPM90HAM is on par with the RG-art, at least on the ones that I played.