Advantages of Inheritor:
Big bonuses to two stats.
-10% to rank gains, but we gain the ability to raise rank on our own terms rather then needing to seek out super-strong opponents. Very valuable for people who like rank but don't like taking big risks!
New magic system! Either the stuff the Magus used (re-read that fight and tell me his spells aren't cool as heck), Archer guy's clone spam and space-time control, or we can have Vanrier hang around and get an OK-ish power. Also, To Shatter Heaven is applied, giving us 400% advancement rate and the ability to break any limits the powers have, and we start out with a substantial fraction of the original user's skill - no taking five days to reach some proficiency, we have a lot right off the bat!
Downsides of Inheritor:
-10% to rank gains (but see above).
Mental Contamination - this is the big one and the only reason I see to vote against Inheritor. It's a valid concern though. I think it's worth the powers, but I can understand if others don't.
Either/Or:
We get to drag the original user's soul along. Good if it's Vanrier, the other two we could just put to sleep and not interact with them. Actually, wait, we could interrogate them for info about the Temple! And wasn't the Magus an Inner resident? It might be hard to get him to talk, but if we could do so that would get us vital info about our enemy's fortress!
Also, I need to sleep, so I'll be leaving soon. I'm glad we survived.