AI is much harder although it may depend on the complexity of the project.
Infact, AI tends to be one of the hardest and most complex piece of programming in games today, People EXPECT smart AI so games (specifically those that are not sequels and have to build the AI from scratch rather than look for stuff they can reuse) have to have smart AI these days.
It's not like the old days where in pacman, the ghosts would work on a node based system and just come for you when your in their range or where enemies in sonic would just do certain attacks/move in a certain way based on timing and stuff (although some of that could be considered more pathfinding than attacking AI, still AI anyway). Back then you could get away with not having teh smartest AI ever but these days, as systems get more powerful, people expect everything (from graphics to AI, to even lagless networking multiplayer games) to get better.
That said, I'm not as knowledgeable with multiplayer technology so my answer may be biased but I do recall hearing professional game devs cite that AI is one of the hardest parts of game programming today.
edit: Oh and I know you said your talking about RTS games but what I said could be applied to most games today, not just RTS', just thought I'd mention that