Since you use mainly swords and are looking for a sidearm gun, I recommend the "Mixing Weapon Types" section of detailed sword guide.

My actual advice is that you upgrade Volt Driver and Mega Magnus, and ignore your other guns. Supernova has its uses for crowd-control, when used well. But antiguas are not great at damage, crowd-control, or anything else important. Of course, you should try a wide variety of guns, because trying weapons is much of the fun of this game. But don't expect anything to beat alchemers, magnuses, and autoguns for damage output.

If you still want to upgrade your blaster, it depends on what weapons you'll be using it with. How many weapon slots do you use: 2, 3, or 4? You want to mix damage types. If you really can't choose, then do either normal (for versatility, against all targets) or elemental (because that's the single best damage type against turrets, which can cause sworders trouble). This is all in my sword guide.