Used both think the spell damage may just edge it as it adds to all spells I believe..

Like most things in ESO the answer is really "its complicated". Things to keep in mind is that the spell/weapon damage enchant has an uptime of 50% (5 seconds on with 10 sec cooldown) while the damage enchants have a 5 second cooldown.

If you were just spamming a single spell at a single target with a half-decent DPS build you'd probably find that actually the damage enchant will do a little more damage over a period of time. Of course this almost never happens. When you start using multiple DoTs and damage spells then the spell damage enchant quickly starts to overtake the direct damage enchant and it gets even better when you attack multiple targets.

Theorycrafting the exact details for this comparison is complicated and generally not worth it as it depends on a large number of factors. Best thing to do is to actually make both enchants and try each of them for a while and see if you can notice a difference. I'm guessing, though, that unless you do very tightly controlled tests over a large number of trials you won't see much difference compared to the normal DPS variation you usually see.