Medusa(Lancer) or Lartoria (Alter) Help

You can't really compare the two.

Medusa Lancer main point are:
Reduce enemy quick resist (interlude) with NP also chance to stun
Charm enemy
also has guts and invincibility in one skill

Lartoria Alter
Is a AOE lancer with skill seal debuff
Very strong Crit damage with all of her buff activated.

It's really the question what you have.
Though For Single target Lancer there are better options so maybe Lantoria alter

baber alter lancer is one of the best lancers in the game even at a 4* and even more so if you grail her to 90, if you look at it in a pure attack stand point.

she also has the best mana burst at 55% added to her buster, and if you combine it with her second skill that sucks up stars and increases her crit strength, she packs one helleva punch. her charisma skill is lacking, but it's skill an attack buff to her alread awesome attack stat.

medusa lily isn't necessarily bad, great survivability, she just is severely outclassed by baber alter lancer imo.

Lartoria Alter is one of the best Lancers in the game. She is great at both AoE and ST damage due to her first 2 skills and her NP can both curse and NP seal all enemies. She can be an amazing addition to your team.
Ana in other hand is one of the worst Lancers in the game until her NP interlude, and even then.she is very niche. Her skills are bad, Siren Song is not guaranteed even at lvl 10, her Monstrous Strength only lasts 1 turn with a low value and her NP gain is ok but nothing amazing. After her NP interlude she becomes good in a quick team to buff the damage of other better attackers, but she is still not the best option. If you have a Quick team she will be decent in the future but so will Fionn so i'm not saying much

Lancer Alter, and by a pretty wide margin. Medusa Lancer just isn't good right now, because of her low NP gain and damage. She has surivability, but she doesn't do that much even if she lives. Her main selling point is her NP's stun, but with how bad her NP gain is it's difficult to use her in that niche. Lancer Alter on the other hand hits like a truck, and has not only a buster NP, but also is a strong crit lancer due to her second skill. So overall, unless you're planning on stunlocking a boss into oblivion, Lalter is probably the better choice.

Medusa lily is my waifu and I love using her, but her damage isn't that impressive. Jaguar warrior's np definitely outclasses Medusa, at least at np3.

Latoria definitely packs a much stronger punch and is the better gameplay choice, but she is not my waifu.

Simple, if you dont have lily at least NP2, then forget it just use lalter