Minecraft – Is it possible to build a Survival Games server that has multiple games running at once

From the About page it seems like it has lower idle CPU levels, optimises some of the memory uses, and heavily reduces network usage. It sounds like it uses its CPU cycles better, but doesn't necessarily reduce them when it's not idle, in order to give higher performance at the same CPU usage.

Apart from guesses from the About page though, you can only really try it and compare. If you do, you can post an answer to your own question.

Spigot is a fork of CraftBukkit and implements the same Bukkit plugin API, and is designed to be a drop-in replacement for CraftBukkit.

The advantage of Spigot is extreme configurability. If you know what you're doing, you can see a 50% drop in RAM and CPU use. But that's a big if – since you're asking, you should probably assume that the extreme configurability of Spigot will cause you more trouble than it's worth right now. Spigot is kind of like the "Enterprise Server" version of CraftBukkit, intended for expertly-tuned dedicated servers.

Spigot does update slower too, since it depends on CraftBukkit development.

On the plus side, since they're compatible you can always start with CrafBukkit and then move to Spigot once you feel like you understand how to manage a Bukkit-based server. Believe me, there is plenty to learn just in setting up a "basic" CraftBukkit server.