I have 2 friends with that ka m4 (at fcsurplus for 299, not full metal, nylon fibre, very nice)
and 1 friend with the ics sig, also 299 at fcs.
Both guns are great, i like the m4 better, feels/looks great, but really i'm sure you'll be fine with either.
Also, the guy at fcs actually recommended an ics m4 over the ka because of what you said, the split gear box.
just trying to confuse you a little more!
Go with your gut!