PDW's and Caliber choices?

I saw a story recently about HK "possibly" introducing a semi-auto MP7 pistol for us mere mortals.

This got me thinking about the question of "What is a good, effective, semi-auto PDW?"
(*PDW for this discussion being a larger-size pistol, carbine or SBR capable of holding 20+ rounds, semi-auto only!)

Personally, I just don't "get it" with regards to the ultra-small cartridges like HK's 4.6x30mm or the PS90's 5.7x28mm.

So, for what I'm interested in, what would be the general pro's and con's of the following cartridges, in terms of 10-100 yd. effectiveness, ammo availability and general "usefulness?"

4.6x30mm- IMO, not "sold" on it's effectiveness, proprietary, not reloadable, expensive, only 1 firearm so far... maybe.
5.7x28mm- Pretty much "same as above" but I honestly don't know if it's reloadable or not, only 2 choices for firearm so far.
22WMR-  Good availability, "might" be decent for HD if you're concerned about overpenetration, but also not reloadable. Not many choices for firearm.  (Threw this in for the heck of it, a friend has a PMR30 that he likes!)
9mm and .45ACP- Proven record of effective hollow-point ammo, LOTS of ammo choices and various carbines and pistols to use, reloadable!
.223- LOTS of ammo choices of all kinds, reloadable, many pistols, carbines and accessories to choose from.

I own a Kriss SDP .45ACP, and a DB-15 pistol in .223. They have both been completely reliable so far and I'm going to be getting more range time with them now that the weather is finally cooperating.