The Homing Amulet subweapon is the main attraction. It allows you to maintain damage while not even paying focus to aiming directly, thus allowing you to instead turn your attention fully to dodging. Obviously, this has a lot of its own appeals. However, it is the weaker weapon of Reimu's options, and it also has its own limits in crowd control because you can't finely control the shots even with focus/unfocus. When enemies are weak enough for the amulets to destroy instantly, it can clear the screen, but against the tougher fairies then you will find the damage lacking. Against bosses, remember that you'll be fighting them for longer so you'll generally get a lower Spellcard Bonus in addition to having a large time for error on attacks.
The Fantasy Seal spellcard unleashes eight large homing orbs that whirl about to strike your enemies. It can do a large amount of concentrated damage, and also has a fair duration as a result. Overall it's a very solid spellcard.
Persuasion Needle fires a wide wall of needle shots forwards. This is less a "Damage" setup, more of a "balance". It is Reimu's more powerful one, but it is also wide-ranging while unfocused, allowing you to switch between incredibly concentrated damage versus a spread of damage. The unfocused shot is great for clearing out waves of enemies, while switching to focused to take out big singular threats is always an avenue that is easy to switch. Against bosses, you will defeat them faster due to the much more powerful damage, but you must be aiming as a result. Whether or not this hampers your survivability is largely based on the Spellcard you're facing.
The Evil Sealing Circle spellcard causes a wave of energy that strikes the screen. It doesn't do a lot of damage, especially on bosses, but it hits everything and that's really its main purpose. It's the weaker of Reimu's spellcards if you want to put a finger on it.
Conclusion - Reimu-A is geared towards survivability. The homing shots let you focus on evasion while the bomb is powerful enough to blow through pretty much any threat. Reimu-B is geared towards scoring. The damage is more powerful to end things quicker, and you'll instead be saving your bombs for scoring opportunities where the duration/strength isn't as much of a factor in comparison to the coverage.
One extra note, and something you might want to get adjusted to. In many Touhou games, Stage 4 tends to differ based on your character choice. In EoSD in particular, Patchouli's spellcard selection differs not just on which character you picked, but on which attack form you picked. The overall difficulty of her spellcards in this particular pair of matchups doesn't change too much, but they are different enough to be noticeable.