In terms of sheer precision, the spindle upgrade alone should get you a bit of an improvement. (If only from improvements in the collet taper) Not sure how much it’ll matter but there are other benefits to the spindle.
The ball screws are also just subjectively better in most regards. Again though, this improvement is probably not relevant if you’ve been happy with the belt anyway.
Sounds like both would be good upgrades but neither are strictly necessary. Still, new toys you can write off… 