LegalForce: I wondered if anyone knows the difference, in broad terms, in distance between a ball that's stated distance is 5.9 vs 5.6 vs 5.5 vs 5.4?
In very broad terms, a ball with 5.5 distance (for example) will add about 5.5 yards of distance per 100 yards, if you hit it flat. But, in addition to the landing area effects that Cicero mentioned, the spin you put on the ball will affect the distance as well.
If you're already used to the free promo balls, the L13 Srixon balls (200 cr) have the exact same specs.
If you want to try something else out, my suggestion would be to try some of the lower credit balls first. The low-level Callaway balls can be had for 250-300 cr. The low-level TaylorMade balls can be had for 400 cr. And the TaylorMade TP5x (L81+) at 550 cr is an excellent ball. IMHO, at your skill level, there is no need for you to spend $10-15 (i.e. 1000-1500 cr) on a ball. You'd just be throwing your credits away.