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I guess if you are just getting started and you only own two guns, you could make a case for both using the same ammo. But you should still stock up.

Thunder08 is right. One, two, or twenty guns won't do much good if you don't have ammo. The type (rifle or pistol) that will be available to you is the type that you stocked up on.

When you consider the cost to stock up. 9mm starts to make a lot sense.

Pistol rounds speed up considerably and pack a lot more punch when shot from a 16" barrel.

Just as a quick reference, from the May 2010 American Rifleman is an article on Ruger 327 revolvers. At the end of the article is the usual 'Shooting Results (25 Yds.)" table. However this time there are two pistols so you get to see the difference in a 5.5" barrel on the Ruger Blackhawk and the 4.2" barrel on the GP100 for four different types of ammo. I'll just pick one type from the table as an example - American Eagle No. AE327 100-Gr JSP:

Vel at 10' and energy for the 5.5" = 1725 fps and 660 ft-lbs
Vel at 10' and energy for the 4.2" = 1580 fps and 554 ft-lbs

This just a 1.3" difference.

Maybe there a few of the CHLforum members that know how to access specific ballistic data for different barrel lengths.

I like the idea of a pistol carbine for home defense because they have more power than pistols in the same caliber, you can shoot them very precisely (even under pressure), have lots of shots in reserve with a large capacity mag, and you don't have to worry about shooting through multiple walls (like with a .223/5.56). And the CX4 is great choice.