I have used both.
I used a GoldTouch for a year and a half or so which my employer bought for me, but then went to a new job and requested a Kinesis Advantage and they got it for me. I have since purchased two Advantages for myself (so I provided my own at a subsequent new job). In fact, I think I also own a GoldTouch but now that I'm thinking about it I can't remember where it is. Maybe my partner took it to her job to use (she also has another for her Mac, which has a slightly different key layout).
The Advantage is amazing. I remap a couple of the keys to work better for me (for instance, I always pressed the wrong arrow when moving up and down, so I just swapped them. I also remap CapsLock to Escape as I'm a Vim user). However, it is expensive. The footswitch(es) that you can get are also pretty cool but I've never really used them much. I really like the fact that the key columns are straight up and down; one of the things that really started bugging me about "normal" keyboards (incl. the GoldTouch) was that my left hand always felt like it had to move at a severe angle to go from the home row to the top row of keys.
The GoldTouch is also great and much less expensive. I always wished that it could be spread more but as it was it did a great job of alleviating the RSI strain. I really liked that they put they put some "action" keys (home, end, pgup, pgdn maybe?) on the left of the keyboard.
Both of these suffer from a lack of a number pad. That's rarely a problem but annoying when it comes up. Both have a built-in number pad but using it is less than stellar. At least, I think the GoldTouch has a built in one. Maybe I'm misremembering.
I use(d) both of these as Dvorak keyboards, but I don't spring the extra dough for the legends since I never look at anything but the shifted numbers.
I also got some ErgoRest arm rests. Those, too, are great, but some desks are hostile to them.