I had both phones for a brief time... and... I much preferred the AD2000s.
But... I think the GS1000s might have been defective, or may not have been adequately burnt in, or may not have had the right set up... or... something. Don't know, but they just didn't sound as good to me as the AD2000s, or my RS-1s.
I definitely didn't like the fit of the GS1000s - too loose, not close - which I think hampered their sound. I noticed they sounded much better when I pressed them to my ears - more like the RS-1s, with a bigger soundstage and more refined highs. But their headband spring isn't strong enough to hold them tight, and I wasn't about to do that with my hands all the time. In my mind... they have some major design flaws - which prevent them from being great phones.
Between the AD2000s and RS-1s... I think I prefer the AD2000s - though its close. They sound very similar... except the AD2000s have a bit larger, airier soundstage, and may be a wee bit cooler. A very tough decision between the two - except - wood instruments seem to sound a bit better with the RS-1s (may be placebo effect, imagined). Though, I much prefer the fit of the AD2000s.
Quite frankly... until very recently... the AD2000s weren't well known on Headfi. They're now beginning to be recognized for their truly exceptional qualities - and are considered among the best dynamic phones available (somewhat Stax like, with bass), very similar to the Ed9s. And a major bargain.