It depends on who is asking and what the time period is.
Modern howitzers (or gun-howitzer or whatever you want to call it) can perform better than traditional "guns" because "guns" are generally low angle (low number of angles, really) and that means you can hide behind a hill and yeet them big since you can do plunging fire and a gun can't, because guns have fixed charges. The difference between a modern howitzer and a modern gun comes down to the propellant more or less, which allows for more angle trajectories (or "zones") to be fired at a given elevation of the bore. A gun can only fire a pair of zones, usually, with one ordinary zone and one low power zone.
Dedicated guns are only preferable insofar that, because they are generally higher pressure/bigger than most howitzers, they typically have longer firing ranges. But this isn't really inherent to the gun design: 2S7, a field gun (with a high and low pressure charge), still gets out shot by G6 or M777ER, both howitzers with modular, multiple zone firing, though. You could totally make Pion a howitzer with a modular charge and it would be better for it. But that would cost money, which is something Ukraine does not have, and it needs to shoot over the Minsk ceasefire line.
Incidentally the M110/M107 combination was supposed to turn into an all M107 force, but eventually itself turned into M110A2, using lessons learned from the M107 gun explosion problem (the barrels were popping after 400-9338 [yes] rounds in Vietnam, despite having a rated life of 1,200 EFCs or something), mostly because the M110A2's 8" projectile was easier to turn into a nuclear shell and there were more 8" shells available: the reason M110 existed at all is because the US Army had been using the 8" gun since before WW1. Much like how it had been using the 155mm M107 projectile (not to be confused with the Gun, M107) from the Korean War until very recently. Ultimately the preferable solution is a single type of weapon, but people will probably continue to use 8" guns well into the future, because it's cheaper to dig something out of a depot than it is to fly all the way to South Africa and ask a factory that probably doesn't exist anymore to give you a howitzer that they forgot how to make. But why is 8" so common? In France, in America, in Russia, in China it's all the same caliber? Nothing special, at least about the caliber. Britain just made a boatload of 'em and gave them to anyone fighting the Kaiser. Everything else followed.
tl;dr The ultimate field artillery piece is basically a 6-6.1" howitzer (again, this is nothing special, France gave a lot of 155mm pieces to everyone [Russia included, naturally, and the USA] and depending on the conversion factors used ended up as 152mm or 155mm) with a high maximum operating pressure. You can call this a gun-howitzer if you want. Or you can call it a Gun+, or whatever, although since genuine howitzers (i.e. the short barreled, shrimply chodes like WW2 mountain guns) are still being made and shot at people the world over, so I imagine that gun-howitzer has more of an informative impact. It's a gun, but it shoots like a howitzer.
Perhaps we can rename howitzers like L118 to "Mortar-Howitzer" or something? That might only apply to 2B9, though, since he is both breech loading and excessively high angled (his breech does not travel much, of course). Truly the moritzer comes of age. But basically the ideal mixture is something like a 2B9, but maybe less magazine fed and more just fairly high operating pressure for certain shells (so what you'd call a "gun-mortar" but I'm calling it a "mortar-howitzer") for short-range all angle fire (including direct fire) and towed by trucks [this replaces the 120mm mortar, the 105mm howitzer, and other things], a gun-howitzer that shoots at as long a range as possible but uses modular charges for lots of angles of fire [this replaces the all field tube artilleries in general from battalion to corps], and possibly an excessively outsized mortar (subsonic low pressure) for maximum high explosive payload and plunging fire.
Galla doesn't believe in the last one. It prefers the 16" rifles it liberated from a battleship that washed up on shore once.
