You can't go wrong with either one. Pearl white is beautiful. It's even the official Bonnie endorsed color I believe.
The argument for Sig red is you can't get it again on the Model X. You could always get a white Model X many years from now if you replaced it with a new one. And it's beautiful as well.
Says the man who's had a White Roadster and a Sig Red Model S. Does anyone smell bias?!