Lumia 900 is the best choice. The build quality is amazing. Its not painted on colors. Its White, Blue, Black to the core. Meaning if you dented it, the core color wouldn't be some random color. Also, screen replacement is easily done. Sim card tray looks pretty cool too....
On the Samsung, the SuperAMOLED display is nice and all, but since its a windows phone, the tiles are always in the same spot, the tiles after a period of time, burn into the screen. You can see this on all Samsung devices with SuperAMOLED. Even though the Focus has more PPI than the Lumia, the screen on the Lumia looks much nicer.
Since their processors are both practically the same, performance shouldn't be a problem. The battery life on the Nokia is great. I haven't tested the Focus 2 for too long, but i had a good look at it briefly.
IMO, i wouldn't go pick up a Samsung made Windows phone. I feel as if HTC's, Nokia's, and other brands have more quality built WP7/8 phones out there.