You bring up a great point. If you want to kill everything for the xp, then yeah, bard is slightly less quick at murder than a highly focused gloomstalker. Lore bards help a lot with cutting words and can drop faerie fire for you teamates to capitalize on, but that is just support stuff not raw damage.
If the choice is solely based on xp, my next question would be: are you planning on doing as many side quests as possible, or at least a majority? If so, I wouldn't worry too much about xp, you max out about halfway through EA if you do most things, and I suspect if there wasn't a cap we'd be halfway to 7 by then end of EA.
If you are just going to play through the story, then aiming to kill everything in your path (or most everything) is probably worth it.
I wouldn't necessarily call myself a "completionist," but I generally will look to do what I can to gain XP in any particular area before moving on. There's time's when that becomes tedious, though, so I just forge ahead. But in general, I pay significant attention to side quests.
Another aspect of this is... for the Devotion Paladin... I genuinely enjoy the "lawful stupid" aspect of the prototypical "virtuous" and "honorable" paladin. But I do it in a sort of goofy way, I suppose. Take Sturm from Dragonlance, combine him with Carrot from Terry Pratchet's discworld series, and toss in a heap of the paladin from "Gamers: Dorkness Rising," and there you go. But my question is, can such a character WORK in BG3? I mean, I don't see that character taking anything but the most straight forward "kill the bad guys" approach to things... So, when you need to go and kill certain enemies in the middle of other enemies, this guy is probably gonna go, "Ok, we'll get to the main guys after we carve through these guys in front of us... no poop on my face, no real even TALKING to these bad guys... they're clearly bad, we're here to put an end to ALL OF THEM..." I mean, is that even a feasible thing to do in this game? I've been in a few situations where you just got mobbed so quickly... I think this might be why the devotion paladin is in third place... I just don't see how he'd be ok with sneaking through an enemy encampment or *pretending* to be allied with them even for "the greater good"...
Thoughts on that one?