It can pierce infinitely and its aim animation will catch up to where cursors are quicker compared to the Lunar Flare. This is because the Lunar Flare has a delayed attack, the magic dealing sprites must first fall from random points in the air and move towards where targeted. Aside from response time the Lunar Flare's projectiles do not pierce mobs, they collide, resulting in hitting fewer mobs per second.
Perhaps there are people who find it useful in controlled setting but in general usage the Prism is certainly more practical since players most commonly fight in wide, unobstructed places.
Due to Last Prism's wide spread and infinite piercing, it's better for crowd-clearing and enemies with multiple segments.
Lunar Flare however can outdamage Last Prism on single-target enemies (such as most bosses), and can go through walls.
So I'd rather have both and switch between them as the situation demands.
But in terms of design, I prefer the Last Prism, so I voted for that.
I didn't find the mana requirements are big a problem as I expected. Using the Celestial Emblem and Nebula Armor, I found I didn't really to rely heavily on potions at all. I'm not saying that I can keep a continuous beam going full time, but that's overkill anyway.
Getting Mana Sickness when outfitted in full Nebula Armor means you're doing it wrong.
Get rid of the Mana Flower, replace it with Celestial Magnet/Cuffs/Emblem, and start picking up mana stars and the blue buffs. You can manually use Mana Potions if you absolutely need to anyway.
If I had to choose one, I'd probably go for Lunar Flare. Nice and simply point-and-click spell with great results and pretty good mana usage. Last Prism is amazing if you can keep it going(and does pretty well in short bursts over a wide area), but the mana drain bugs me to no end in extended use.
Among all spells though, I'd say Razorblade Typhoon. Great item for Moon Lord/Pillars and farming in general, and not having to aim is always nice.
This is one of the few times I think I'd ever say "Who honestly cares"
Both come at the same place; when you've completed every challenge there is to complete. Both utterly curbstomp everything by design, they are postgame superweapons. There is no situation in which you'd ever need to analzy which one does what better, because they both do the job in overkill.
Definitely the Lunar Flare... you point, click, and ignore it for about two minutes until it runs out and needs to be restarted. Matched with the dragon, you pretty much don't have to do anymore fighting when you don't want to... which is about 90% of the game. Who wants mobs messing with you while fishing or building or exploring?
I tried that prism once... didn't like it, never used it again. Pretty worthless to me for how I play the game....