I like the first one better -- it has more punch -- but it doesn't say that Evelyn is also an AI, which is what I get from the second. And I'm not seeing an "or else".
Could you mash them together, sort of like this:
When a supercomputer enslaves the planet, AI teenager Evelyn is sent defeat it (or else what?)
If you could squeeze in why Evelyn is chosen, that would be great. But I think it's more important to get the stakes for her in.
With Twitter expanded to 280 characters, I'm not sure how it will effect the contests. Maybe try for a short one and a longer version, so you're prepared no matter what the rules become.
Best of luck!
“We must always take sides. Neutrality helps the oppressor, never the victim. Silence encourages the tormentor, never the tormented.” ― Elie Wiesel