I consider Elemental damage to be more useful than Shadow damage. This is because you don't want to get too close to Constructs (especially Gun Puppies and Retrodes) and Undeads (especially Phantoms) who are weak to it. You won't use Kilowatt Pulsar to kill Beasts and Gremlins (except for the Flamethrower Gremlins) anyways because they dodge your bullets so any gun is pretty useless against them. Shadow on the other hand is strong against Gremlins and Gremlins dodge (with the exception of Flamethrower Gremlins) so this makes Toxic Catalyzer useless against an enemies that is weak to it.
==Edit==
However if you are familiar with how Gremlins dodge your bullet, you can kill them with Toxic Catalyzer with ease.
And you should get both Catalyzer and Pulsar (if you are really not sure which one to get) like I did.
Well, there's a pretty big difference. The pulsar shot transforms into a more damaging explosive shot that splashes and does knockback after it travels half its' range.. plus a chance for shock if you go with the elemental variety. Go with that if you want to spam explosives all over the place from a distance.
The catalyzer, it charges almost instantly and fires out a shot that does no damage but "sticks" to the enemy (orbits around them, to be more precise) and then once you hit that enemy with a regular shot the charges explode, doing a lot of damage in a good sized radius, also with heavy knockback. And you can stick multiple charges to one guy if you want to do some really crazy damage. You get your choice there of dark damage with poison, or just slightly higher normal damage.
Generally speaking the Catalyzers are weapons you'll either use by themselves or to fill a gap where you need both a gun and a bomb. The only thing they really combo well with is Freeze weapons, because you can stick the charges on a frozen enemy without breaking them free, wait for them to take damage from thawing out on their own, and then blow them up. The problem with the Catalyzers is that if your teammates are all stupid they'll feel the urge to always kill whatever you've just tagged with the explosive, and that completely wastes it. You can only detonate it while the monster is still alive.
The pulsar, on the other hand, is the opposite. Your teammates won't mess you up, but since it knocks monsters all over the place YOU can mess THEM up. Of course, only about as much as ANYBODY WITH ANY SWORD does, so I try not to feel too bad about it. ;)