I don't take the upgrade and keep the ability. I don't do basic attacks that often and my Archer kills...alot. I get my knockdowns from ES. I almost never stop spamming Longshot so when I am not using that I mostly use Leapingshot, I don't have time, nor do I want to waste time, to use an extra basic attakc whenI can kill them with a Longshot before they get close
In Perilous it doesn't really matter which way you go honestly. As I said, whatever floats your boat.
For Nightmare though, the thing is you get way too much aggro with Longshot. You can't hold spawn points reliably either. Case in point, when we play together, I'm most of the time the last one standing as an
Archer. It's not because I'm that good, it's because when I aggro something, I aggro only that thing and I kill it fast.
Furthermore, longshot is spreading most of its damage amongst multiple targets. Don't get me wrong, it's very good damage, but not enough to kill outright in 3 seconds. I get as much kills as a Reaver while I use "single" target Leaping Shot almost exclusively. My experience is that if you have 3
Archers using Longshot the whole game, things get dicey at times. Get the same
Archers using Leaping Shot and it's a breeze, things simply melt. Not only that, but the clearing time by killing everything is at least 25% faster using Leaping Shot vs Longshot.
It's only my opinion, so take it with a grain of salt. Both abilities are viable in Nightmare, they simply need very different playstyle.