I am conflicted....

As you can see here: Yellow fires at a much higher rate of speed, though his aim is a bit low.

Looking at Mr. Green, shown here: you can clearly see that he has a head shot. Now i know they are both made entirly of heads, but his is more of a head shot. Yellow's is more of a chest shot. Taking into consideration what curse said about the rail gun, I think that even though Mr. Green's gun appears to fire slow bullets that is just in bullet time and in reality Yellow's head would be pierced before he could fire a single shot.

Mr. Green PWNZ J00