To use ball expander or straight?

[stradibarrius]

I have a hornady .308 FL sizing die. It has a teardrop shaped expander and a straight. Which is best to use? I'm shooting steel out to a 1000 with a Sako TRG .308

 

[want2learn]

there's a little discussion of this topic on Accurateshooter.com (https://www.accurateshooter.com/technical-articles/reloading/expander-mandrels-and-neck-tension/)

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Mandrels vs. Expander Balls on Decapping Rods
If you haven’t acquired an appropriate expander mandrel for your brass, but you DO have a full-length sizing die with an expander ball, this will also function to “iron out” the necks and reduce tension. However, using a die with an expander ball will work the necks more — since you first size them down, then the ball expands them up again. Typically (but not always), run-out is worse when using an expander ball vs. an expander mandrel.
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[spife7980]

So something to clarify, you are using a full length bushing die, not a regular full length die if it came with two decapping rod accessories. As such you can control the amount of neck sizing via the bushing so an expander may not always be necessary. So in addition to the expander ball you also have a decapping pin retainer, not a second expander ball. That decapping pin retainer should never touch the brass, if it does then you have a bushing that is entirely too small.

So to answer which is best requires you to provide some actual information. And when you collect that actual information you will find your answer.