INC x,n adds n to x to create a new value of x. x=x+n also does the same thing. I think INC is easier to use when you set linear integer variables (like x = number of enemies killed), and x=x(any functions) for calculating things that increase or decrease non-linearly.
Not sure about any speed difference though, but I think it'll depend more on what the changing variable is doing in the program than how n is added to x. I'm curious to know as well. Maybe try this.
q=0
do
INC q,1
if q = 1000 then end
loop
and
r=0
do
r=r+1
if r = 1000 then end
loop
Run them and time which one ends faster? Is this silly enough to work? Not with this code literally, but with some sort of advanced tomfoolery...