Personally I prefer to shoot my subguns vs M16 (or variant). Cheaper, and easier to control. IMHO.
However if I had to get a 223/556, then it would be a M16. With modifications, you can shoot other calibers. Only annoying thing is that depending what you want to shoot, you may need to disassemble it. For example if I want to shoot my Razorback, I need to pull out my Geissele SSF (install a lighten spring with a milspec F/A trigger), buffer, buffer extension, etc. Just a pain. If your talking about a Shrike belt fed (or MCR) then the pain is to replace the bolt release with a nonstandard one due to it contacting the upper, as well as other swap out parts..