Tough choice! Space Gray is a very nice color and a safe color. It's safe like Titanium Silver but with a little more wow to it. No one will ever say they don't like your color choice and resale will be easier with Space Gray but....
Montego Blue is quite striking. The pics with the masking looks too soft but the other pics LeMans posted look much better. I will say this though: If you get Montego Blue you must butch up the car a little to overcome the slightly more feminine nature of the color. I say this as a guy who used to own a Zenith Blue Boxster which appears to be in the same range of blue as Montego. (See attached.) Some bigger wheels and tints would make it look even better.
My E90 color choice was made easier because I ruled out Sparkling Graphite due to its closeness to Steel Gray on my X5 and I had to have Terra leather so that ruled out some colors I thought would look bad with it. I really thought about Monaco Blue but it was so close to black and I didn't think it was as nice looking when paired with Terra.
I know, get one of each.
