So our group is starting up a short-run ascension game, and I'm going to be an Imperial Psker/Temple Calix Psykana ascended to Interrogator. I was curious whether I should build up, or down? Each has its pros and cons.. but is there an established preference? Example: building up means each extra wound costs 200, but building down means there only 5 (albeit cheap) wounds to purchase.

I'm a bit worried about measuring up to the rest of the characters. So far we'll likely have a Psyker inquisitor, which means Unnatural Willpower yielding ridiculous psychic ability, and a Vindicare Assassin.. unnatural agility AND Untouchable means no psychic powers can touch him and he'll be a dodging machine.

My force-weapon wielding (probably temporal and telekenetic) psyker interrogator is going to pale next to them in combat, I feel. Of course, it isn't all combat.. but the Inquisitor will likely have social skills as well.

First, your question:

Top down, as it lets you have access to both branches of imperial psyker, and therefore psy rating 6, and therefore much easier discipline mastery.

On the other hand, the rules for top down might make you have to pay 500xp for both WS and INT advances, depending on your interpretation of the rules, in which case I'd definitely go for bottom up.

Second some thoughts about things you mentioned in your post:

I don't see how you're going to measure up to these other two characters in combat. Psykers with Unnatural WIllpower are quite formidable, and Vindicares are, well, Vindicares. So you should prepare yourself for the eventuality that you will be outshone in that aspect of the game. You might want to consider asking your GM to allow you to ascend to Crusader, since you're Templar Calix and that fits with the feel of the class, and would help you keep up with the others.

I'd also check out some of the minor powers in the Radical's handbook. They'll help you shine in the non-combat aspects of the game.

Lastly, although I wanted to mention it first. I don't believe there should be untouchable Vindicare Assassins. There is a specific temple, Culexus, for Untouchables, and since all assassins go through a common basic training before being assigned to a temple, I think all the untouchables would be diverted there, making them unsuitable PC's. Your GM may not have been made aware, or made an exception, or I could be wrong, but it seems wrong to me.

(As always, I'm willing to be corrected on the fluff, if I'm wrong.)

normally I'ld prefer bottom up... more wounds and you don't have to buy the most expensive version of a specific attribute

psykers on the other hand can get very much from the top down build which are already mentioned by The Boy Named Crow

The Boy Named Crow said:

The Culexus Temple employs Pariahs specifically and exclusively, and it's possible that not all Untouchables are Pariahs. See wh40k.lexicanum.com/wiki/Blank for some info. I'm not saying you're wrong, rather that it can be interpreted both ways. I'd personally not allow the Untouchable Vindicare, but others may disagree.

@The "what am I supposed to do?" question
The Inquisitor will be the social-one, the Vindicare will be stealth&security... how about being Investigation&Intelligence?

The Interrogator has a lot of "Choose-Your-skill"-Things going on (as far as I remember). Talk to your GM about what value knowledge of "Tactic"-Knowledge-Skill will be and what Investigation skill he would advise for being valuable. Talk to the other players if they are willing to leave this to you.

If you GM is willing to churn out boni for stealth, Initiative, surprise-checks etc. if you come up with well layed battle plans (beefed/backed by checks for Logic, Tactic, Navigation(land) and other skills) you are still of the "martial value" one would expect from a psyker hailing from an battle orientated order.

my two cents

Thanks for the help.

I certainly would have had the option for Crusader, but I called Interrogator as soon as I found out that we'd be making characters for a short-lived romp through Ascension.

I suppose I might go down then, for access to PR 6.

My only problem is that i end up buying all the cool stuff and then miss the basics! Ugh. Maybe I should just do a test build, going up, so I know what essentials I should take.