Still Alive from Mirror's Edge sounds like a vaguely generic inoffensive pop song. Still Alive from Portal is a catchy but simple song that is genuinely funny in context. I wouldn't call either of them masterpieces, but I guess the ME song has a slight edge in technical song complexity, but that doesn't really matter imo.
The Portal song is way more iconic and serves its purpose in the game better as well as having done a much better job resonating with the general public 39 million views vs 9 million views (on its most popular video) on YouTube should be enough to show that one of those stuck in the minds of people much more strongly. I'd hazard a guess that when reading the topic title most people immediately thought of Portal until the OP reminded them the Mirror's Edge song existed and was named Still Alive.
It's gotta be Still Alive (Portal) by a mile for me.