Just take polaris

Polaris (elemental)

Pros: -elemental damage is good vs. undead and constructs
-has really good knockback effect when shooting from far away
-can interrupt gun puppies
-perfect as a standalone gun if you normally use mostly swords
-can cause shock, which is a powerful status that occasionally interrupts monster movement and attacks

Cons: -elemental damage is bad vs. gremlins and beasts
-shock damage can send quicksilvers into spazzout mode
-knockback can be REALLY annoying to party members ***** (to the point where it has earned the nickname "Trollaris")
-recipes for upgrades are in high demand and are only available from Basil or for a ridiculous price on the AH

Biohazard (shadow)

Pros: -shadow damage is good vs. slimes and gremlins
-interesting charge mechanism that can do explosion damage to nearby monsters
-easy to hit "dodgy" (like gremlins) enemies with due to slow bullet speed

Cons: -shadow damage is bad vs. fiends and undead
-slow bullet speed can be annoying and takes getting used to
-recipes for upgrades only available at Basil and AH (but not as high of demand as Polaris on AH)

You decide^^ Keep in mind the regular bullets of Biohazard do not stick... only the charged shots!

EDIT: Thanks Lord! lol good eye...

Polaris is an utility gun meant for shutting down turrets and herding enemies into corners. It does elemental damage and shock, meaning it deals effective damage against 2 out of 3 turret types. It also causes a helluvalot of knockback, enough to cancel attacks. If you're careful with it, you can learn to kill turrets with it, without them firing once. Unfortunately, this gun also causes rage if used wantonly in parties, as it can knock enemies into your allies if you're not careful with it. It's invaluable when running solo - just be careful when playing with other people, and don't spam it.

Biohazard causes explosions. Explosions are epic. Shadow-damage explosions that poison stuff are even more epic. It is excellent at taking out large groups of jellies, although it takes some skill to use. In large parties, you may find that by the time you have set up charges for a massive explosion, your teammates have already killed your walking bomb. It's a fun gun to play with, and the charge time is very short in comparison to other weapons, meaning that you can set up several charges at once. I'm not sure whether Gremlins will dodge the charge shots or not, unfortunately.

I find the Biohazard to be a bit more specialist than the Polaris, to be honest. Personally, I'd prefer the Polaris as a first option.

(Since you mentioned you want something for 15 Bark Modules, you might want to take a look at the Static Capacitor, an excellent status haze-type bomb in its own right that can deal a lot of damage to groups of undead or constructs... but that's entirely your decision.)

Well, I have fun with my catalyzer, but polaris is just better, there is no reason to not use it over catalyzer