Servant Comparation

Well it depends, they serve different roles, Tamamo is great to use against male bosses, Karna on the other hand has an aoe np so his purpose is more set towards farming. I personally feel that Karna would be better since if you ever need a st np servant there’s probably a Scathach somewhere already on your friend list or maybe even a Bryn, who also has an advantage against against a good chunk of males(even without that advantage she’s still a hard hitter), Aoe lancers simply don’t seem to be as common(at least based on my friends list) but hey, if you already have a Liz or something in terms of Aoe lancers, go for Tamamo. Really it’s a matter about who between the two you lack the most.

It is dependant on who you like and what your team needs, but I personally prefer single target np servants. They are useful for bosses and can handle most farming nodes without too much trouble. Not to mention tamamo's inherit abilities, stats, and attributes make her really strong even without her np which is great against male archers as a niche. Her demerits can be a pain, but I would pick her over karna.

If you really want a general answer: Karna is the best. But a general answer is the worst kind of answer. Tamamo having an ST NP and buster heavy deck is excellent at killing individual targets and thus brave chains. Karna though has some of the highest damage one can get out of the lancer class. Not to mention he's a crit magnet.
I used to prefer ST np servants on the merit that an ST np is just far more useful on boss fights but there's more to it than that. The servant that changed my mind is also an AOE lancer like karna, Titoria. Though her np is underwhelming as it's AOE (although it is spammable) the rest of her kit including her damage and survivability make her very useful to me even in non cleave damage situations.

Funny I have them both aswell, but ended up leveling Tamamo Lancer over Karna. My team has too much AOE np, so Tamamo was the way to go. I like her kit better than his aswell. Plus in the next event she will help you more. Since its all about female servants.