I would get the 18.5", personally, for several reasons:
-2.5" makes a difference in velocity
-I'm not a fan of the M4 profile, particularly since I don't own an M203
-I don't know what brand SI Defense is using, but Krieger is always a good brand, even in their less expensive line.
Of course if length is crucial than you would want the 16", but you probably wouldn't even be asking if length was that crucial.
I have a DPMS with a 18" barrel and it's a handy length.