| Powerline networking is slow, from what I hear. If you're already willing to get wireless adapters, don't even bother considering powerline.
Do you expect to do any heavy sharing directly between the computers upstairs? Like streaming music or videos? If that's the case, I'd use the bridge so they can be wired directly to each other, it'll be faster and support the streaming better. That will also reduce the amount of shared wireless bandwidth they eat just talking to each other. I imagine they'd be pretty easy to set up, at least as easy as a wireless router.
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