If you mean domestically, I think you need both. Some Departments have carbines and shotguns in each patrol car. I think that is a great idea. Then have a few SMGs for the SRT.
I think that the pistol caliber SMG is the better choice for entry teams. Muzzle blast, penetration, controllability, etc. being the reasons.
For the military, I think the M4 or similar carbine is the best choice 99% of the time. Special Ops requiring something like the MP5SD will always be better served by SMGs.
Personally, if I had to choose between the two , for all possible situations, I would want the MP5 for my full auto weapon. It could be surpressed much easier and you almost don't notice it moving in a 3-shot-burst. I don't think I would need the extended range or penetration of the carbine. To be honest, I would want the G-18, for the extra concealability/portability.
Truth be told, I will probably be buying a MAC-11 next year with a suppressor. I can't justify spending the $$ for an MP and I can't get a G-18. Anyone have a better suggestion for under $1500, please share!
[This message has been edited by Rob (edited 01-05-99).]