[3.5/PF] Torn between two Builds

[MunsterJujus]

So I have a character idea I came up with but now I'm torn as to how to build her. (Using Level 10 as a benchmark because I might end up using her in a campaign coming up.)

Basic story: When she was little she made a deal with outsider/spirit and now it follows her around as her teacher/assistant. It give her advise and helps her but cannot physically interact with the world. She tries to avoid fighting as much as possible; instead she tries to help people in need. If there is no way for her to get out of a conflict peacefully the spirit possesses her. The problem is the spirit sucks at possession, he causes her to turn feral and rage out until all the threats are destroyed or until he can get her to safety.

So back story done I can't decide between two builds:

Invulnerable, Wild Rager Barbarian 5 / Synthesis Summoner 5 - This gets me the helpful caster stuff but also the freakout rage machine during possession.

Or

Chirurgeon / Beastmorph Alchemist 10 - This has more of the happy healer tied to it and the freakouts would be pure fluff (Possession would be using a mutagen).

I like the first cause it fits what I want the possession aspect of the character to feel like. The second one is more fitting to how I want the character to act while not in combat.

Do you guys have an ideas that could help with my dilemma?

I only used PF because the concept came to me during a study break for my finals and I didn't want to stop studying to delve through my books. So if something in 3.5 works feel free to list 'em.

Thanks

[JadePhoenix]

Well, Fiendish Codex I has rules for (demonic) possession, but this could be easily used by a different creature. With Leadership or a similar feat and a nice DM, you could use whichever build you want.

But I would definitely use the Barbarian/Synthesist build if that is not an option.

[stack]

Rage won't help a synthesist. You get the eidolon's physical scores, rage only buffs the base scores, which are entirely replaced. Plus the "suddenly possessed" thing doesn't really work with a 1 minute summon time.

[Psyren]

Your 1st build is illegal - you can't combine Invulnerable Rager and Wild Rager as both replace the same class feature (Uncanny Dodge.)

[SamBurke]

Problem is true again with the synthesist and stackability, but ask your DM, they might make an exception: for example, have the rage be a class feature for the eidolon/spirit and not you. It's a disadvantage (you can only use it when the spirit/eidolon is summoned), and much worse than rage, so it might be a decent tradeoff.