Both are kinda bad. Get nitro or shiver.

.. you like to solo from time to time. For soloing, you need a bomb that does serious damage. I can waltz through tier 3 wolvers' dens with nothing but this bomb. It also helps significantly in those large deconsctrution zones, which are full of soft-bodied gremlins.
P.S. I carry a 1) Combuster, which hits with only 5% less damage than a Leviathan (10% less than the clunky-slow DA), but does significantly more damage due to setting the enemy on fire, 2) DA for certain levels, or a Leviathan, 3) DBB bomb, 4) Argent Peacemaker, which is far and away the best gun.

@guyjin

My Hail Driver would like to have a word with you Guyjin.

@op

I can't comment on the differences between DBB and RSS, being a gunslinger. However I've found that the Twisted Spine Cone is really nice for wolver danger rooms in T2. I'd imagine that once I finish getting enough heat to upgrade it, it may outright replace my Leviathan Blade for all but boss runs.

(Since I have a Hail Driver for Constructs/UD, a Blitz/Callahan for fiends/chromalisks/wolver-only groups, a Biohazard for Slimes and gremlin-only groups. The only time I even really have trouble in an arena are wolver/gremlin rooms, thanks to the massive differences in strategies for kiting wolvers and gremlins. So far I haven't found a good gun for those danger rooms. I think the DBB may be my ticket.)

Don't get a bomb for the sake of getting a bomb, really need to consider your goals for acquiring a new weapon to decide if a bomb is even the right option. Especially if you already have a bomber in your party and you aren't wearing any CTR reducing armor. A bomb's DPS is directly linked to its CTR.

From your description it seems you need something to deal with fiends and beasts. Your only piercing weapon is the Strike Needle which difficult for these type due to their dodging reaction. You might be best off grabbing a Flourish/Snarble Barb and see if that will do the trick. If you are really set on RSS/DBB here's what I think on the two:

RSS - no worry about being 4*. The damage penalty doesn't kick in till the last strata and even then it still does respectable damage for those depths. This bomb does massive damage but only if placed perfectly. It'll knockdown/interrupt enemies but due to the projectiles will activate dodging. T3 Fiends can be dreadfully annoying when they start jumping around all over and you have to get uncomfortably close to get multiple shards to hit. T3 Wolvers are much easier since they tend to take damage prior to teleporting, but still bothersome.

DBB - Used almost as one would a blast bomb, so quite natural. If chained it will immobilize wolvers and devilites. Also gremlins in T2 are pretty easy with this bomb. The shield on gremlin backs in T3 make it less effective but still works well enough. You can use this bomb to soften up the horde then go in for the kill with your swords.

In the places where I would use an RSS or DBB extensively I honestly prefer the DBB:

Wolver Den - DBB's AoE softens up the group as a whole, kiting is trivial with wolvers and you can lay every bomb to hit the silkwings which finishes them off quickly. The RSS works here too but only wolvers hit by the shards take damage and are knocked down. The silkwings can be difficult to get to with an RSS since they can move around alot or be completely surrounded by wolvers.

Devilite Level - DBB so they don't dodge and I can hit many at a time while maintaining my distance. Just keep moving while charging, don't want one to pellet you.

Gremlin/Wolver Danger Room - This is a tough one since both of these bombs excel here. I still feel DBB has the edge though due to the AoE.