Altera or Mordred?

Altera has crest of the star while mordred has secret of pedigree. Altera is better in most cases. That said, if you need a saber I highly suggest you skip out on the singularity banners and wait for a better saber. We do get a free np5 saber for halloween and a free story locked saber for camelot clear. Of course, if you are a new player and need a saber right away you can just roll for mordred as waifu is number one prio. Trust me, you can work with any servant you have, especially if they are a 5 star and once merlin comes about, any buster servant seems better.
TLDR: roll for mordred or wait until halloween

Roll for your waifu. I guarantee your game will became more vivid with team made of characters what you sympathize + you going to have more reasons for effective farm because of this
(im also going to roll for Mordred and Lalter durinning London pickup summon)

Altera does not have instinct and that's really all you need to know to find out who's better. Memes aside Altera has one of the highest attack stats in the game, a strong 3 turn attack buff that also provides a burst of stars and will get an interlude that let's her second skill raise star absorbtion for one turn letting her pull in all the stars Crest creates.

By contrast Mordred can farm marginally better thanks to having a NP charge skill but is otherwise worse in almost every way due to having a small NP charge skill but her NP hits less hard too and she has no support for others while Altera has tactics(which though Altera's weakest skill is still something... And also not instinct).

All in all unless you really, really want to kill lancer Artoria then Altera will be the better choice(and even then Altera will be almost as good due to having better normals).

Basically, Altera is better for raw attack, Mordred is better at surviving in longer fights. It kinda comes down to what sort of team you have/are wanting to run generally speaking. But, as said prior, just focus more on the Servants you have a vested interest in. That interest can lead you to getting far more mileage from them than normal. i.e. I used Kiyohime on my frontline even through E Pluribus Unum despite having arguably better Berserkers because I had a team around Kiyo to cover her weaknesses. tl;dr always focus on your interests for Servants

Go with waifu. This game is more than just stats and numbers. Your servants have dialogue and personalities that you learn. I've learned things about Jalter only a Master would know (unless you cheat and Google it) and that alone makes it special. Also its freaking Jalter Haha best avenger waifu.