The Roarmulus guns might look superficially similar because they're both explodey, but they're worlds apart in both purpose and technique! The Pulsegun is very defensive, newb-friendly for solo play and newb-unfriendly for parties. Catalyser is very offense-oriented and mostly for experienced gunners. There is one big non-superficial similarity between the two, though; in a party either Roarmulus gun can be awesome or awful, depending on whether you learn to use it expertly. They both have an udeserved reputation due to so many peops judging them by suboptimal use without consideration for what dedicated users have gotten out of them. :(

For newbies, Pulsar is a great weapon to use to clear tough missions in solo if not in parties. I think the closest comparison is the Blaster, which I recommend over the Pulsegun if you're focusing on guns. If not specifically focusing on guns, it can go either way due to uniqueness of each players's interests, aptitudes and aesthetics. For purely practical purposes, I think a Blaster is more useful, esp. if you're focusing on the gun class: compared to Blaster, Pulsegun is more convenient against turrets and much safer vs. Rocket Puppies, but those are some of the strengths of the gun class in general and Alchemers in specific. Inside the Pulsar's minimum range, OTOH, Blaster offers better damage and knockback, making it one of the most defensive guns for uncomfortably crowded locked rooms. Combined with Magnus-like projectile speed and almost (not quite, I think) Antigua-like mobility and spammability, and decent interrupt power, Blaster is IMO the most all-around defensive weapon available. (Those fast projectiles are also useful against Beasts, Devilites and Gremlins since they can hit before dodging happens, but for dedicated gunner the Needler is more desired for those monsters.)

Catalyser is one of the most offensive weapons, which is probably not what most newbies can most benefit from. It was one of my first guns and I've enjoyed it greatly since, so YMMV. Purpose overlaps most with Autoguns, though techniques are much different and the Cats are more AoE than single-target focused and more general-purpose than specialised. If you learn how to use a Catalyser, the Shadow version's much more powerful than the Sentenza, useful for when you want a Shadow gun but Umbra Driver's ricochets would be a problem. The Normal versions's like the Pepperbox in that it's a handy substitute for high damage when the abNormal damage type is inappropriate, but IMO the Catalyser is better at this due to greater flexibility (Un-charged attacks are still decent and single-charging rather than stacking charges allows rapid flinch infliction.) and lack of Fire to weaken it against Oilers.

Static Capacitor looks like one of the most useful bombs, so unless you just don't enjoy playing with vapourisers, that's the top choice for a newbie: it's useful in solo and in parties, and is pretty straitforward in its use. Just drop it in monsters' path and indulge in some schadenfreude. If you're new enough to be choosing your first Roarmulus token reward, I recommend Static Cap if you often play in a party, especially if you go PUGging. If you almost always play solo, Pulsar's the most practical new tool. If you often play in a very familiar group, Pulsar can still be easy to use IF the party tends to stick together very well and play very defensively. (For example, imagine swords being used for knockback rather than damage, just pushing monsters away when they get too close and retreating to the safety of the gun-spamming party. Pulsar's newb-friendly for that playstyle.)