There are several issues: 1) effectiveness of slab vrs. framing/crawl space, 2) effort/cost, 3) storage use, 4) future considerations
1) Both systems are effective and are widely used for radon remediation. The key to both systems is sealing the soil area from the living area.
2) The even if the cost is not important, the effort to level the existing soil sufficiently to pour a slab is tremendous.
3) The most important factor may be what is going to be stored. If you are storing Christmas decorations, then that load is minimal (estimated at 30 lbs. per square foot) and wood framing may be appropriate. However, if you’re storing stacks of books, (estimated at 150 - 200 lbs. per square foot) a concrete slab is more appropriate, which will easily support 500 - 1,000 lbs. per square foot. A wood framing system will support 200 lbs. per square foot, but the additional framing, footings, etc. need to be included now not later.
4) If you’re not sure what the space will be used for in the future, I’d definitely recommend a wood framing system with a small crawl space so you could add electrical wiring, plumbing, heating ducts, etc. in the future.