New to Android, Nexus 5 on Straight Talk a good idea?

Hey all,

My GF is plunging into the smartphone area, and I have encouraged her to consider the Nexus 5, based on its high end quality and specs but nice low price. I'm an at&t Lumia user myself (though I use an Android tablet) so I don't know much about Android phones nor much about connecting an unlocked Android phone to the Straight Talk network.

Or, does Straight Talk suck and I should look elsewhere?

Or, does the Nexus 5 suck for a new user and I should look elsewhere? Moto X? Used Galaxy?

Any and all advice would be helpful at this point. We need around the Nexus price point and the Straight Talk price point, but any varying suggestions are welcome. And do I need a certain Nexus phone to get it on these bring your own phone networks?

Thanks for any help!