I'd pick the St Regis over the ITC Grand Central.
Location of the St Regis is much better. It's part of Phoenix Mall which has a movie theatre, lots of shops and restaurants. It is also a 15 minute drive from the Sea link. There are close to a 100 restaurants within 1 Km of the St Regis. There isn't much around the ITC other than discount shops. I also prefer the St Regis restaurants to those in the ITC.
What do you plan to do while you are in Mumbai? If you can split your stay, consider spending a few days at the JW Marriott Juhu. Traffic in Mumbai is really bad with a road/metro construction going on everywhere so if you have a specific itinerary in mind try staying at a hotel that will minimize your drive time.