How to equalise highly asymmetrical rise and fall times?
Your rise and falls times aren't all that asymmetrical. Sure, the rising edge has that long tail as it approaches +5, but that really doesn't matter, since you are driving a digital input. What counts is the time from either 0 or +5 to (approximately) the mid=point, and from the looks of the scope traces that is a few tens of nanoseconds. If you need better symmetry you're probably better off with a higher-speed comparator. 

So let's start by dropping back to basic questions. How much symmetry do you need, and why?