managing color - Photoshop vs printer

[lakevostok]

I'm sure this is a beginner's question, but... I'm using a Canon Pixma MP950, printing out of Photoshop on Canon's pro paper. When I choose to have Photoshop manage colors and select the profile for the printer/paper combination I'm using the results are WAY off. When I simply let the printer manage colors I get results very close to those on screen. Why would this be? I thought that by choosing a profile that actually matched my printer and paper I'd get more accurate results, not less. Also, if I send this image to a professional printer will their results be more likely to match the 'printer manages color' version (that looks like what I see on screen) or the 'Photoshop manages color' that's giving me such off-base colors?

Any help would be greatly appreciated.