I need to buy a new seatpost and am considering a Thompson or something comparable.
I don’t want to just replace cheap with cheap. (not that you care)
My question is what is the advantage of the setback post (if any) If I can get the same position with a straight post.

It's not an "advantage".

It's about how you fit your bike, it corresponds to your KOPS and your femur length. If you like to move your butt (and knees) further back, get the setback, vice versa.

There is no advantage to a setback seat post if you can get the correct position with a non-setback post. There is one disadvantage: unnecessary weight.

I read somewhere that the advantage of a Thompson setback seat post (where the post itself is bent) over a traditional setback post (where the clamp is offset) is that it is lighter in weight (the lighter, bent post replaces the heavier, offset head to achieve the displacement).

The only reason people use these is that they need a slight adjustment to their riding position that their frame's geometry won't allow.

Unless you're having trouble with your fit now, I see no reason to get a set back seatpost.

Most seatposts are setback in regard to the clamp area. Thomson just puts a bend in the post and uses a straight clamp.

Truth.

Look at your current saddle position. Is the saddle as far back as it can go? If so, you should probably try a setback post. If your saddle is clamped to your post in a more forward position and you feel comfortable with that, then stick with a straight post.

Yup.

I ordered a Thompson setback a few days ago for this. My bike fit is perfect. If I would gotten the straight seat post it would've positioned my seat too far forward.

Thanks all
my old post is setback by way of the clamp with my seat near the front of the rails.
I think I the straight post will work just fine.

Dyno Drainpipe...who else wanted one?

I have short legs and a long body.

In order to stand over my bike, I ride a frame one size too small.

In the case of my Pista, a professional fitting revealed that a 35mm setback seatpost would properly place me over the pedals and the bike in general.

On my Karate Monkey, with 175mm cranks and a Cane Creek Thudbuster, I ride one frame size too small and everything fits without a setback; but then, the Thudbuster tends to place the saddle a little further back than a normal seatpost would, and the Karate Monkey has a more relaxed seatpost angle, and so, when extended, the seatpost goes further back than on my Pista.