No, it's much less than 50%. It's more like a flat ~80 more damage. Nova Driver to Valiance is a terrible comparison, they're completely different guns with a different clip size and everything.

My effectiveness is hindered by my eclecticism, as I can't resist trying all sorts of stuff instead of focusing until I can reach FSC where the plentiful crowns are. :(

By using the Kozma weapons given for free at 2-stars, you can get a feel for which weapons work best for you against which monsters, keeping in mind that some guns and probably all bombs are "late bloomers" compared to swords; they get much, much better when you hit 4-stars. From experimenting with those 3 freebies, you can decide which combinations of class and damage type make the most sense for you. As long as the weapons you choose have at least one that you're comfortable with against any given monster, you're good. If you happen to prefer the same weapon class for every given monster, or for any other reason want to be a "pure" sword, gun or bomb user, the choice is likely very simple; Meatloaf the damage types! (as in, "Two out of Three Ain't Bad.") On another hand, it is entirely possible that mixed-class focus with one or more Normal-damage weapons is ideal; bonus damage for abNormal types is great, but not really necessary for viability. Being able to freely use any of the classes you like against ANY monster "anywhere, any time" is also really nice. It's a matter of personal preference; there are lots of viable options, but mileage varies so that I think the increased availability of the 3 "basic" weapons is a wise game design choice.

For example, I am most interested in bonus damage vs Fiends due to Greavers and Devilites, and Greavers move fast enough that I find the sword best. For Devilites, this is overshadowed by my preference for guns vs. Devilites, so I need at least one that's not Shadow type. As it happens, the Pulsar is ideal for me vs. their great numbers and their particular dodging behaviour, and the Greaver-destroying sword is an ideal backup for when I have to fight closer than I'd like. Were I the betting type, I'd wager my donkey that Pulsar and toothpick is a very popular combo for general Clockworks use. Mecha Knights take a bit more patience to gun down, but it's mostly not really difficult or dangerous.

As another example, I like an Elemental sword to take care of Mecha Knights and don't happen to like any of the Pierce guns as "main" weapons but really like the Catalyser and prefer guns vs. Gremlins. Here, I love how both Brandishes and Catalysers have really useful normal AND charged attacks, and the catalyser charge gobs stick to frozen monsters without thawing them. At 3-stars, I can freeze a monster with the sword, stick 3 gun charges, and fire to detonate them without losing the thawing damage!

That's two whole loadouts for me, or one for some one with all 4 slots. If I had to focus on just two weapons, it would be something like one of those combos, except that I am drawn toward the Normal Pulsar instead due to preferring guns over swords against Gremlins and for the latter I think I should have gotten the Fire Brandish first instead of Freezing since it works more smoothly in pick-up groups and I find the full chain-push effect of the charge attack immensely satisfying.