Wow! I just went through this very dilemma when I was designing a custom frame for myself, and IF Steel Crown Jewel (which I should be receiving in just another week or two!). In my case I already knew where I'd be wanting the saddle to be located relative to the bottom bracket center, so I started by figuring out where the seat post would need to be in order for the seat post clamp to be right about mid-rail on the saddle (I already knew what saddle I'd be using, in my case a Fizik Alliante). Then it was just a matter of figuring out what seat tube angle would make this work using a straight seat post and a set back seat post. It came out that there was something like a 1.5 degree difference in seat tube angle between the two scenarios.

So what difference does a couple degrees of seat tube angle make? Well, as you relax the seat tube angle you reach a point where in order for the seat tube to clear the rear tire by a comfortable margin, you'll need to extend the chainstays. Chainstays are a source of flex in the bottom bracket, and increased flex can lead to a less responsive and less efficient frame, so keeping the chainstays short is a good thing if you're going after frame efficiency (none of this matters if you're building a touring bike or something like that, though). Since I want my frame to be an efficient frame, I wanted to go with fairly short chain stays. This meant using a somewhat steeper seat tube angle, and then compensating by going with a set back seat post.

My new frame with have chainstays that are 410mm long, a seat tube angle of 72.5 degrees, a gap of right about 1cm between the seat tube and the rear tire, and I'll be using a Thomson setback seat post (16mm set back). Had I wanted to go with a Thomson straight seat post then I'd have needed a seat tube angle of around 71 degrees (or 71.5 or so if I didn't mind the saddle being all the way back on the seat post), and would have needed chainstays of around 420mm in length in order to keep that 1cm gap (the numbers on using a straight post might be a bit off, I rejected this approach as soon as I realized how long the chainstays would need to be).

In your case, since the frame is already built then which type of seat post you go with has more to do with what saddle you're using (how long the saddle rails are) and how much set back you like your saddle to be at relative to the bottom bracket centerline. If you like saddle back and are using a saddle with typical length rails (like the Alliante) then you'll most likely need a set back seat post. However, if you're using a saddle with really long rails (an older Selle Italia Flite comes to mind) and like to be a fit forward relative to the BB, then it might turn out that a straight post will work for you. You won't know for sure till you build it, but if you need to buy your seat post now then going with a set back would probably be the best bet.

Basically, which seat post you use depends on the frame's seat tube angle, the saddle rail length, and how far back you want your saddle to be.

Hope my rambling helps.

John