If you can afford it, you won't regret going with the 9 ft option.

In our previous house we did an addition with a basement and went with 10 ft ceilings. Never regretted spending that extra money. We had a large light well (required by CA code) and the basement got lots of natural light.

Is you basement a walk-out or regular? What are the plans for windows?

Agreed, go for 9ft in basement, especially for a new build. Nobody wants a troll cave.

Absolutely 9'. If your budget and water table allows it, it's a no-brainer decision. You will pretty much have a minimum 8' ceiling everywhere, even under beams, ductwork, and pipes. For a gym definitely coordinate MEP to allow a 9' ceiling.

The generation that only had and used basements as "cellars" is dwindling. The modern house is defined by basements being utilized for living space.

9 foot is best. Consider when you finish it that the ceilings will be lower because of duct work, beams, etc.

Why does 9 foot ceiling necessarily mean that it gets reduced by beams and ducts. That certainly could vary by convention.

100% agree if you plan to use it as living space, go with 9'. It's surprising how much that extra foot adds to the overall feeling of a space.

I deliberately chose 8 for my cellar, but that's because I never plan to use it as living space. I love having the laundry room and all mechanicals/HVAC so easily accessible in a cellar - and cellars are expected in a home in my area of NY. So yes I wanted a cellar, but the cost to bump mine up to 9 wasn't worth it, to me. YMMV. :)