Portrait will give you less distortion at the edges, because there's less edges. (For instance, Minecraft actually has trouble calculating distance, and removes distance visual effects like fog as it fisheyes the landscape out at the sides. I can post screenshots if you want.) But if you're playing a game that properly renders those aspect ratios, then that's actually a bad thing.
MOAR VERTIKAL REZOLOOSHUN is also better for productivity applications, although I suppose that's not a consideration for most Eyefinity setups. (It was for mine, actually. But I'm weird.)