I got the vfc m4 and its just awesome, solid as hell, good internals/stock accuracy, good compatibility. no complaints at all, except that my wires are a little too long in the back and its hard to cram them in there when my stock is not extended.
compared to the SIG its not even in the same league.
my buddy has the SIG and its okay but overall quality is less. Also not as good of an outdoor gun as the barrel is quite short.