Also, these two guns are of different damage types, and hence not really comparable. Right now your sword is elemental, so you'd be foolish to make your gun elemental as well (except in certain situations, such as FSC, where you can get by with only elemental weapons). If you really want to stick with DA and you really want to use only two weapons, then there is no question that you should avoid Polaris. On the other hand, if you're willing to try another sword --- piercing or shadow --- then get an elemental gun such as Polaris, because they are great for turrets, while swords are not. See here.

Normally I'd advocate mixing damage types. But where the polaris is concerned, there are extenuating circumstances:

1. As long as you're not facing quicksilvers, having a gun that deals shock is a huge asset against crowds of monsters. It also makes your gun more useful in lockdown.

2. The polaris is an elemental gun, whereas the supernova deals normal damage. Thus, the polaris will deal roughly the same or better damage to every monster, except for beasts and gremlins (which resist elemental attacks). But as you go into tier 3, beasts and gremlins become quite hard to shoot, particularly with such slow bullets... and since the bullets on the supernova are just as slow, the normal-damage gun wouldn't help you much with elemental-resistant monsters anyway.

In other words: the polaris is a great gun for the casual player, and whatever liabilities it has originate from sources other than the elemental damage type. I'd recommend doubling down... or, just stick with the 4* version. Most of the crafting costs come at the 5* step, and using 4* gear where possible will allow you to assemble a broader set of more specialized gear.