Instead of comparing apples with apples, now let's compare oranges with oranges: what are some decent Sega Mega Drive-based heavy metal/music that uses guitar-sounding synths? Struggling to think of any. Ones that come to mind include:
The final boss music of Phantasy Star IV (and a good one)
The final boss music of Shin Soseiki Ragnacenty/Crusader of Centy (sounds a bit rough to me but a valiant attempt nevertheless)
Skitchin' (not heavy metal but still worth a listen)
May or may not be considered metal but some tunes from Langrisser is worth a mention and Noriyuki Iwadare would show his metal side in the Grandia series later.
As for Super Famicom/SNES, we can easily name entire soundtracks that fit the theme. Some of these are obvious metal soundtracks with the lead guitar playing the main melody. Other soundtracks have a few token metal songs and/or has the guitar (e.g. bass, rhythm guitar) taking a back seat but the style/sound is still prominent enough to be considered metal:
Rockman X series (X3 being my personal favourite SFC metal)
Super Donkey Kong/Country series (e.g. "pirate" metal final boss theme)
Iron Commando: Koutetsu no Senshi (composer is not Japanese interestingly enough. Or maybe he is Japanese in nationality but not ethnically. Then again, Japanese isn't a race [insert long-winded explanation here] - you know what I mean - on further research, this game isn't even Japanese? Even though it was only released in Japan? Or is it an unlicensed game?)
Battle themes from Final Fantasy USA: Mystic Quest
Battle themes from Breath of Fire II
Battle themes from Estpolis/Lufia II
Battle themes from Romancing Saga series
Battle themes from Rudra no Hihou (noticing a pattern?)
Nobuo Uematsu always been a fan of metal (e.g. Final Fantasy VI's battle themes)
Certain battle themes from Live-A-Live
Last but certainly not least, the final boss theme from Super Mario World 2: Yoshi's Island! I wasn't expecting the epicness of One-Winged Angel as I fought my way to Final Fantasy VII's final boss. Similarly, I wasn't expecting to be greeted with guitars in a game filled with cute Yoshis and colourful picture-book graphics.
Heavy metal? SFC is the clear winner I think.