Ah...that is a really hard question...so far, I think Starcraft is the most balanced RTS existing thus far. Each race requires completely different tactics, but none are at a particular advantage or disadvantage on any given map. Warcraft 3 on the other hand, requires the same tactics for every race...with the only variation being in the combination of units people choose, but essentially evey match is the same (usually a 2 min rush). But however, as far as costom mapping goes, warcraft 3 in infinatly more versatile.
Ive played over 700 games of Starcraft and only a little over 30 WC3...but if I had to choose a game, I would say WC3...simply because of the costomization and mapping.
In starcraft, if you loose your entire army in a blotched attempt to take out your enemy, you can replace it quickly and easily enouph to still stay in the game, whereas in Warcraft, if you loose your army and particularly your heroes in the first major encounter, you've already pretty much lost the game since you never have enouph time to revive your heroes by the time they reach your base, and as you all know, heroes are everything. That is one thing I really don't like about WC3...that and they NEVER STOP PATCHING IT!! One day I WILL be able to safely save a replay without worrying bout it being invalidated the next month.
-VGsatomi