Simba. It's open-source, from the client to the scripts, so you know what's running.
RSBot has taken a new approach and stayed closed source this time and also made it so you no longer deal with compiling scripts and simply use their repository.. Although it is nice, I don't exactly like that much, given the fact that they're now owned by gold4rs. I avoid running powerbot on any windows machines because of this.
In terms of just a bot, powerbot is an injection client after all, and can do things simba can't, and with other things do them a fair bit more effectively.
In that way it's superior.
But Simba can also do thing's rsbot can't either. You can write your own scripts to bot on private servers whereas rsbot cannot do that as it is only for runescape. You can also use simba for pretty much anything outside of runescape also.
The SRL community makes the rsbot/powerbot community look like a joke. But it's more of an educational forum, full of scripts to learn from and many tutorials. Leechers are almost non-existent. Everyone contributes. The powerbot forum is more of a hell-hole.
That being said, if you want a bot that just works without too much thought, rsbot/powerbot.
If you're willing to learn and contribute to an amazing community/project while botting, simba.