I, also, own over half of the options in the list... and IMO, the M-16 is the easy choice:
#1. Modularity. Yeah, I *can* put my HK sear in a belt-fed (e.g., HK21) but it's
harder & much more expensive than, say, putting a Shrike or FM-9 belt fed upper
on an M16 lower.
#2. LOTs of M16 spares, repair parts, accessories. Vs. the HK system, where I've found
hosts can be hit-or-miss; also, you can in many cases buy from the same supplier as
the U.S. military (for the M16), but HK has made it hard for "civilians" to get their stuff.
#3. Regulatory. HK hosts will typically, at a minimum, require an FFL transfer;
Vs. an M16 upper than I can purchase from Brownells, Gunbroker, etc. w/o
messing with any sort of transfer... and sell just as easily, if it comes to that.
#4. Cost. Definitely pay a bit of a premium for HK stuff (even the 'cloned' stuff). Vs. you can
buy an M4 factory upper straight from Colt for < $1K
#5. And with the other (non-HK) choices on the list, you're generally stuck with
what you have (e.g., SMG, belt-fed, assault rifle). Sure, the Lage stuff is awesome
for the M-11, but it just makes it a much better "MAC"-- still nowhere near an
M16