So i was thinking of a character concept which could work with 2 classes, but wanted to know which would be more efficient: Wizard or Summoner. The concept revolves around using the Shamanic magic Theorem, having see supernatural and Easily Possessed while looking to learn Darkness and Essence paths first with no sub-paths.

With the Shaman ability Call Spirits, i'd imagine the character can see those spirits he calls with see supernatural, and most likely communicate with them.

Before going into details about what advantages are needed, i should let you know my GM let's us have up to 5 disadvantages with a limit of 6 additional CPs. What i have figured out so far is Gift 2, Magic Nature 2, See Supernatural 1, Increased Zeon Recovery 2…Easily Possessed 1, Feeble 1, Shamanism 2

the idea is unfinished because it depends on whether it's summoner or wizard.

I'd go for a Wizard. It should better represent your concept, and better work with the general idea…also summoner is fine, but remember that summoners have bad magic projection, so if you're going to use a Summoner with The Gift, you should invest on the Arcana Sephira to get the Predetermined Magic Projection Spheres. Also, summoners get less Zeon than Wizards, which isn't good, especially when you're trying to summon spirits around you. Anyway, if you make a Wizard with Predetermined Magic Projection, when you believe you're satisfied with your MA, you could switch class to Summoner, in order to have a more complete character.

If you intend on using Darkness and Essence Paths, try to get up to 6 disadvantages and get natural knowledge of both, since this would help you a lot with ML. Also, I believe than some subpaths could be fitting for such character, such as Blood, War, Death, and Umbral (depending on the specific characterization of the shaman, some of these might be more fitting than others).

Thank you for the feedback Elric, you're a great help as usual. I'll be fleshing out the character over a few days and i'll post here once i'm done so you can see it.

ok, so i want the Damned 1 disadvantage with the following concept:

Must make a Ritual Sacrifice once a week for one hour or temporarily lose casting ability/maintained spells until The Ritual is done. The Sacrifice can be anything living.

what i'm wondering is, what would be a good value of total Presence that must be sacrificed through this ritual for it to be equivalent to the disadvantage?

 I think the "the sacrifice can be anything living" is to "easy". One hour "ritual bug stomping" isn't this much of a disadvantage.

But, for example, a minimal presence for the sacrifice, which a single sacrifice must have OR all the sacrifice combined must have, sounds more like a disadvantage.

 If you had read the whole post, you would have noticed i was asking for total presence for the sacrifice