Ruby. Although FireRed may have better tiles, Ruby has, in many ways, more possibilities with graphics. Well, ignoring the whole thing with .raws only being editable by hex, that slows things down a little. Ruby has an editable Professor background (which I messed up, so now I have to find where it was repointed and edit it again, ugh), the clock, a bigger second tileset (although it does have a smaller main tileset, but for someone like me, that really doesn't change anything). Lots more stuff too.
Not only that, but with FireRed, I have really bad experiences with game-breaking glitches that caused me to quit two hacks (although one may or may not be being remade and one is going to be a game). Emerald...I can't ASM hack or program very well. So I'd have to use the glitchy tools, incredibly boring hex, or make a hack with a ton of glitches.
So yeah, Ruby all the way.