I stayed at the Westin in 2004, and at the Mercure den Charité further north in Mitte in 2006. That's when I noticed the Melia, which wasn't quite finished at the time of my visit. The Westin does have a fabulous location and it's beautiful and elegant. However, I think I preferred the modern design of the Mercure rooms, even if that was "only" a 3* hotel. I haven't stayed at the Melia but it seems it has very modern rooms and is a 4* hotel. (Maybe the Westin has renovated its rooms, and I had an unrenovated one, but I wasn't terribly impressed, although it was really cheap on Priceline, so I can't complain.)

The Westin is very close to the Französische Straße U-Bahn stop that is one from Friedrichstrasse and of course it's very convenient for walking to attractions. But I think it could be argued it's more convenient to be closer to Friedrichstrasse station, as the Melia is, where you change U-Bahn and S-Bahn lines and take trains. So far as location goes, it's really a wash, in my opinion. The other advantage of the Melia is its proximity to Oranienburger Tor, with its numerous restaurant options.