There are three factors you might want to consider. First, as others have pointed out, there is a big difference between the "W" and "St Regis" brands (W is very stylish, SR more refined). Second, you have location, Nusa Dua is a pain in my opinion if you want to venture "out" whereas the W will be convenient for a number of eating spots. Third, you have the newness of the property, but we stayed at the St Regis right after it had opened and it was fine, I suspect you'll have some teething pains but still really enjoy.

The St Regis is tough to beat, but I have to admit that I am looking at the W for a visit next November. It's a shame they don't have more heavily discounted advanced rates (St Regis tends to have a 25% off rate, W is only 10%).