On a recent bike build I had to make a choice between using either a seatpost that was designed for the bike but had a setback of I believe 15mm or an aftermarket seatpost that had zero setback but requires a shim to fit the bike. According to the fit calculator I require a seatpost with zero setback but I wanted to use the original and moved the saddle forward to the maximum it could go to get the correct fit.

Now I'm wondering if I should go the other direction and use the aftermarket seat post with zero setback and the shim?

Ignore the fit calculator and use the "can you take your hands off the bars while pedaling without sliding forward" method to establish saddle position. See:

That's good for setting the seat angle, but meaningless for the fore/aft adjustment.

However, results rule. The right post is the one that gets the saddle where the OP needs it by calculation, rule of thumb or personal preference.

If either post can do that, the tie breaker would be factors like no-shim fit to the frame, weight, cosmetic, etc.

I use what might be described as "modern fit theory" which says that the proper position for a level saddle should be determined by how much weight is on one's hands. Usually that's done as the rider in the posted video demonstrates, starting at 5:50. The saddle should not be tilted either up or down.

I had the same situation...my bike came with the stock 20mm setback carbon seatpost which required me to move the saddle all the way forward to get comfortable. It always seemed that I sacrificed a lot of saddle rail suspension and road buzz reduction by moving the saddle that much. I chanced upon a zero setback aluminum post and was able to keep the clamp in the middle of the rails. By positioning the saddle there, I found the sweet spot for comfort, power and road buzz reduction. I tend to make whatever change I need based on my body and pretty much disregard calculators and formulas, even if I have to spend a couple of dollars to get there.For me, riding keeps me well, it's worth the money.

Depends on Frame STA, and Your Femur length, etc.

I got a Bike, Koga WTR, with a Zero set back Airwings tour seat post
and, now like the Cane Creek Thudbuster seat post with set back Better ..

short adjustment rails of the B17 that was on the bike made it worse
so the saddle went first, then the Post.
Added the illusion of a lower STA, by using a 180 arm crankset (I'm Not a Spinner)
saddle further behind the pedal at 3:00, the downstroke.

You can get seat posts in diameters that match your frame , from like Thomson in the right size .


./.