I've stayed at both properties within the last month. As they are literally around the corner from each other, location is no issue. I prefer the look of Westins, but this one is really nothing special. I have no idea what the suites look like as I was not upgraded. The W was a cooler place. No upgrade there either. The room was nice enough, but I thought the bed was more comfortable at the Westin. There were two big groups staying at the W when I was there. One was a college group that showed up on three buses that made valet parking difficult to get into. I know that is not a normal thing, but it added a frustrating factor to my stay. The other was some kind of meeting for a skin care company. So there were about 100 pretty attractive women walking around the lobby. The one frustrating thing about my W stay was that a previous poster referenced the platinum breakfast. I was not offered this as a SPG platinum. Not sure why.
I'm returning to ATL next month and I have chosen to stay at the Westin, but not for any really big reason.