If "you're a 9mm guy" and you already have a 9mm 1911 Single Stack: then you don't want 10 round 40 mags (or 10 rounds 10mm mags either).
If you have a 9mm 1911 Single Stack: use that to shoot Classic division with minor ammo. You can compete with 10 round mags when you shoot minor.
If you don't have a 9mm 1911 Single Stack gun but you want to shoot in Classic division with a 40, I recommend sticking with major and shoot with 8 round mags.
If you want the best of both worlds and want a 40 S&W 1911 Single Stack gun so that you can shoot either 8 round major or 10 round minor, then you want the Tripp "System" mags that can hold 10 rounds and fit in the box, after you file down the mag base pad (but you had better have a box on hand when you file: because it's a tight fit and there is very little room for error in filing).
Depending on the gun you use: some guns perfer that you load the ammo long to get maximimum reliablity. In this case you are still shooting 40 S&W ammo, but you are loading them longer than factory length. For this you need to use 10mm mags instead of 40 S&W mags. Tripp sells both, and will make the "system" mags for both 40 and 10mm.
... did I cover all the bases?
-ivan-