To the best of my knowledge, most laptops don't use a resolution that's any higher than 1920 x 1200, in fact I believe most are lower in resolution. As such, you don't need as powerful a GPU to push the pixels through.
I game at 1920 x 1200 and for a while was rocking a 5770. At times I saw the GPUs limitations, games like Crysis I had to sacrifice settings such as anti-aliasing and what not to maintain smooth framerates. But those titles are few and far between, most every title I was able to max out completely.
I mention that because the "6990m" and "GTX580m" and even the "590m" wont' hold a candle to their desktop parts. The mobile parts are always slower in performance for obvious reasons. Even the 590m won't compare to a desktop GTX580. The performance you're getting from these "top-end" mobile gpus really compares more closely to a "high-end-mainstream" desktop GPU like the GTX570 for example.
So in short, I'd say you'd be happy with either.
Both are really solid performers for the resolution you'll likely be running.