Neither.

You probably want an elemental gun to go with your shadow sword. For more detail, see my sword guide (especially the first two sections).

Toxic Needle is unnecesary, since a good blitz can kill anything before poison has a chance to act.

The catalizers arent that bad, but now that you said you have a faust...... my decision remains the same. If you still want poison, Dread venom striker might work for you.

Neither, go for a Nova/Storm Driver, AP or Polaris. An elemental gun.
You're a swordsmaster, ergo you need something to hit inaccessible targets.

All of these stated are good at taking out turrets:

• Nova and Storm can both deal massive damage to single targets with internal ricochets, OHKOing turrets, although this takes some skill. They are also good at dealing with large mobs of zombies or Retrodes, and the latter also inflicts Shock which is a useful status when dealing with large numbers of enemies
• Regrettably, due to the bounce, it is possible to unintentionally set off exploding blocks and gib your teammates by accident, trip switches, or alert monsters you didn't want to wake up. This makes them the cause of much rage in IMF. They also have a small clip (2 bullets)
• Argent Peacemaker has a high firing rate, large clip (6 bullets) and fast bullets as well as a cool-looking eagle that deals knockback and repeated damage on the charge, dragging your enemies along with it. In addition, it is highly mobile
• Its damage per normal shot is not that great, meaning you will need to nearly unload the entire clip in order to get a high DPS on the gun, and each shot has no attack cancelling ability or knockback. This may make it less safe in some situations, particularly when you have a large number of turrets to deal with. Its bullets also have a fairly small hitbox
• Polaris is the king of troll guns, with:
  • Expanding bullets that deal extra damage
  • High attack cancellation ability on expanded bullets
  • Shock infliction
  • Knockback
• The gun can be used to shut down turrets completely in the right hands, stopping them from firing. This is an extremely useful way of dealing with Rocket Puppies
• Unfortunately, it is normally found in the hands of morons, who may well knock Lumbers into your face, mid-attack animation. The flashy explosion its expanded bullets create is also a source of much rage amongst players, and is highly lag-inducing. The bullets are also extremely slow, making them fairly easy to dodge in LD

I have heard good things said about Hail and Magma Drivers, both of which operate in much the same way as Storm and Nova (although the statuses have different uses). However, I can't really speak for these since I own neither gun nor their predecessors.

In short: Drivers can OHKO enemies and deal lots of damage to mobs, as well as crowd-control, AP is good for single-target and mobile gunning, and Polaris is a good utility sidearm for suppressing fire/herding mobs.

If you must have one of the two guns you've mentioned, I would find the Needle to be more useful than the Catalyser. This stands doubly so since you're using a Faust as your alternate weapon, so you already have shadow damage in your arsenal. Despite what people say about the Plague Needle, it still does an extremely respectable amount of damage, if not quite so much as its big brother the Blitz.
If it's the Poison status you're after, I would recommend you take a look at the Dread Venom Striker. (I'd say Venom Veiler, but you've already stated you're not a bomber.) The sword is pretty fun to play with, even if it's not the most powerful weapon out there.

I vote for Toxic needle just because the catalyzer can be a bit tricky to use for less experience players (trying to be more noob friendly) and because u already have shadow wep.