I don't have a GW(M)-530, but I own and know a bit about GW-6900's. Three things to consider when you compare the two watches that you might not know yet:
1) Casio's website shows the MSRP for both models as $130 USD. I'm not sure how discounted the newly revamped GWM-530 is since it just came out, but if you shop around you can usually find the GW-6900 for less than $100, including shipping, since that model has been around for a couple of years.
2) If you think you'll be using the countdown timer feature, make sure you're getting the new GWM-530 -- the old GW-530 had the Time Memo feature instead, which many people didn't care for.
3) If you think that you'll ever want to replace, modify or customize the strap on your new watch, the GW(M)-530 uses an asymmetrical, proprietary strap attachment system that only accepts identical replacement straps OR the bracelet from the MTG-(M)900 model.
The GW-6900, on the other hand, uses Casio's very common 16mm lug width, which means you can use any strap from most of the 5600 "square" models, the DW-6900 models, and the popular NATO/Zulu adapters for 22 or 24mm nylon straps. This means that there's a wide variety of colors and styles for straps for the 6900, as opposed to "two" for the GWM-530.
I have a whole pile of 6900's (about ten or so) and two 500's (basically the silver and black and the black and orange version of a 530.) I like both models for different reasons. The 6900's are fun, and come in a lot of different colors, but (to me at least) the 500's seem more refined. Maybe it is the hint of metal going around the bezel. The 6900's are wider, but wear a little thinner (I never measured, just going by the feel/look on my wrists.)
If you get the chance to try both on, definitely do. I picked up my "Fire" 500 at JC Pennys, so I know they can be found in the mall, and 6900's are in a lot of stores. Ultimately, it is whichever you like better, but if I had to pick just one or the other, it would be the 500/530... big digits, easy to use and a really nice look to it. I had a Time Memo version I returned (bought it used and it was not waterproof) that I did not like as much as the newer version. The new ones have a countdown timer, and tick off seconds in the first eye instead of five minutes. There are a few other minor changes, too. You can tell them apart from a distance because the button on the top right is either marked "Time Memo" or "Reverse."
Of course, I started off with a 500, and ended up with 20+ G-Shocks... so you could always buy both of them!
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