I decided to take the bullet and, while I think 10 would've been the better price point, 15 makes sense and honestly isn't egregious. Futo's DLC is actually more like a expansion pack/mini sequel. It has a #(7, I think) of dungeons which share Jinryu Tenketsu's mechanic of getting harder each time you do them, with better rewards each time. Also it has its own little area, which is pretty cute. Bullet Reporter on the other hand is only one twp (albiet pretty and with fantastic music) dungeons that gives you the Aya PC, Aya Partner, and Momiji Partner. It's not terribly overpriced, but it's definitely higher than I'd put it.
As for how the playable characters rank in:
Reimu is probably the best over-all option, and is the yard-stick the others are measured to.
Momiji replaces her 5-cost with Blocking, which severely reduces all damage taken for 1 turn and responds to the first attack taken with a counter. Her 3-cost is also a V shape instead of 3 attacks. She's honestly the worst of the lot, but still has the 1 and 10 costs so not bad. Her 5-cost is also probably the best answer to Big multi-hitters in the game for melee range, so it gives you a safety net if you run into one in a corridor.
Aya's 3-cost is a auto-target that does a considerably high amount of single target damage. Her 10 cost is basically Reimu's but with the added effect of knockback. The defensive option, pretty solid.
Futo's 5-cost works as a chain reaction that splashes onto all touching foes. Her 10-cost is a hail mary to RNG that auto-targets everything in sight and does a varying amount of damage that can range from "whatever" to "holy shit." The best single target damage.
Over-all it doesn't make too much of a difference and you're fine taking anyone. As a fair warning, they all have their own version of Fantasy Seal and All Sides Lethal Dragon so you're gonna have to do some inventory management if you want to do Bring Items dungeons - but at the same time their effects are completely identical to Fantasy Seal / All Sides, so there's no pros or cons there.
Edit: bullet reporter actually has a second dungeon, and it's brutal as heck.