Both + crit

Magicka abilities scale from maximum magicka and spell damage, and are affected by spell critical
Stamina abilities scale from maximum stamina and weapon damage, and are affected by weapon critical
There are also a few other skills that scale of maximum health only, even if they consume magicka or stamina: dragon blood (DK) and arctic wind (Warden) which are heals and gripping shards (warden)
Ultimate abilities scale from whichever is higher of maximum magicka/stamina/health and spell/weapon damage
Flames of oblivion (DK) consumes magicka but follows the same rule as ultimates, scaling on your highest stats

The scaling for most skills damage/healing is based on (10.5 x damage + maximum stat) x scaling coefficient + constant. For heavy & light attacks 10.5 becomes about 40, so spell/weapon damage matter more if you rely on those for DPS.

Damage shields scale only on maximum magicka: harness/dampen magicka (light armor), hardened/empowered ward and pet abilities (sorcerer).
or maximum health: ingenous/fragmented shield (DK), radiant ward/blazing shield (Templar) and spiked bone shield/bone surge (undaunted)

Almost all skills are boosted by the respective CPs.

Yes, the only spot where hybrids shine is PvE tanking, because most tanking abilities use magicka, but tanks need enough stamina to block reliably for extended periods, and enough health to survive hard hits, that do a lot of damage even when blocked. There are indeed some PvP hybrid builds, but one pool is dominant on most of those. For example magicka users put tri-glyphs or use sets that give a bit of stamina, like Shacklebreaker to deal with CCs, sprint, roll dodge. Sets like Pelinal don't really do anything good, since your stats will be still way lower than a magicka or stamina build, even if they are roughly equal to each other. Pelinal also doesn't add that much recovery so you most likely have to invest more in sustain, decreasing damage further.