The underlying color processing is always ProPhoto RGB with a linear encoding (gamma 1.0). You can export that in one of four gamma corrected color spaces: sRGB, ColorMatch RGB, Adobe RGB (1998) or ProPhoto RGB.
I'd use (and do use) ProPhoto RGB because the capture device I use (a Canon 5D) and the output device I print with (Epson 3800 and 4800) have the potential to exceed Adobe RGB (1998) gamut.
See this:
http://www.adobe.com/digitalimag/pdf...p_colspace.pdf