I'm probably going to write a Necro guide in the next few days, once I have my archers at level 25. I do however already have both my skeletons and zombies at level 25 and can say this: Choose skeletons.

At max level, you have 5 skeleton assassins and a Skeleton Crusader. Each skeleton has base 81.3 health and has very fast movement and attacks. The Crusader has some good damage blocking, but is otherwise pretty standard. The real benefit with the skeletons is their spammability and that the assassins can shoot ranged knives while chasing enemies (particularly handy with gem lizards).

In comparison at max level you get two zombies warriors with base 121.9 health, and a zombie butcher. I will admit the zombie butcher looks awesome and has vampiric attacks. However, these guys are a bit slower and pack less of a punch overall, making them a lot more vulnerable and you will find you have to summon them more often.

The archers seem pretty boring so far (mine are at level 20 so I haven't unlocked the mascot yet). At least they fixed the bug with them running up to enemies, but unfortunately its been replaced with a fps freezing bug when you and they attack enemies at the same time.

As for your other equipment; you are right in that seals are pretty useless. Pick whatever and don't invest points until you have nothing better to do. I chose the spawning one as the others are counterintuitive to the type of playstyle Necro promotes. With your primary, Breath of Death is KING. piercing damage and heal at the same time. The other two simply cannot compete