Having used both M16S1 and SAR21 (went through conversion
course, shooting, and had 3 ICTs of revision for SAR21)......
.... i STILL think M16S1 is much much better and user
friendlier...maybe except for the slightly tedious zeroing...
For me, the SAR21 kept jamming on the bullets and digging out the
rounds takes at LEAST 5 TIMES MORE time then M16S1 clearing the
jam....
and I had many an occasion where its because of the jamming, the
enemies run across me and "conquered" my area due to the sickening
jam.... 
i mean, its almost like M16 (single or double feeding, etc) 2-3
seconds VS SAR21 (>1 minutes,
)