I have both a portable AC (the kind with the hose that gets vented out the window) and one that's mounted in the window, so I can actually compare them pretty much side by side (different parts of the house). They are pretty similar in BTU's, (11,000 vs 12,000) but the window mounted one is much more efficient, and much quieter. If at all possible, get a window mounted AC. Only go for the portable one if that's your only option.
Hope this helps!