Will you day trip or stay overnight? Blue is better for weekends away, has more terrain. MSLM is closer to Toronto, if it's day trips I'd go there. Overall I prefer blue due to steeper terrain on north end.
Blue is about half an hour further than mslm. I found mslm to be less busy. I didn't enjoy the short runs at mslm but the conditions there were great last year. Maybe I'll get a mslm pass and a blue 5x7.
Remember that the advantage that Blue once had over MSLM was night riding. MSLM now has lights as well, so if you are looking to avoid the crowd issue, you can always start your day later in the afternoon when the masses head home, ride into the evening when the hill has far less traffic.
Pouts has given you the hill perspective. The question for you is how much do you value the extra vertical and terrain for the extra drive time / gas cost?