With the same wool, bigger needles will give bigger stitches, and a looser fabric. Smaller needles will give smaller stitches, and a tighter, warmer, denser, harder-wearing fabric. The needle size is probably what an average knitter would use to get the gauge (which is x stitches per 10 cm/4in). Some people knit tightly, and they need a bigger needle to get the same size.
Some people knit loosely, so their stitches are bigger, and they would need a smaller needle to get that stitch size. On the recommended needles knit a 6 in. square - of stocking stitch , unless the gauge/tension part of the label states otherwise - and measure your stitch number over 4 in. in the middle. If you get more stitches than the label says, you are a tight knitter, and need to use a slightly bigger needle to get gauge.
The label gauge is what the manufacturer thinks is necessary to get the best fabric: not too stiff, drapes nicely, but not holey. You might want to change it: for socks, you normally knit tighter than recommended because you don’t want nice drape and you do want good wear. Or if you’re doing lace, you might want a bigger needle to get net-like fabric. I say do a tension square with 7s and another one with the 9s, decide which fabric you like best. Make sure you wash the square the way you normally will with the knitted item first. Make sure you know which one’s the 7s, and which the 9s.
Which one gives the best fabric for the item you are going to make? Go with that.
You only need the 8s for this if you think ‘the square knitted on the 7s is too loose and floppy and net-like, but the square done on 9s is too tight/dense/etc, right in between would be perfect!’.