Bigjim68
Head Chef
I have both, and I use the 10 inch far more than any other knife in the block.A lot of the things I cut up with a gyuto (or chef's knife) are large enough that an 8" won't go thru them. Things like squash, heads of cabbage, melons etc are usually a bit too big to cut without sawing or repositioning the product. This is especially true when I'm cutting meat or carving. If you're cutting a rib roast or ribeye steaks (especially if you're using "Ups") then you can't get thru them with one stroke, meaning you have to "saw." This takes more work & time plus leaves marks on the meat. Also things like sheet cakes are too wide to span with an 8" but a 10" will cut them cleanly.
Again, I've already admitted mine is a bias based on cooking professionally. At home it's not a big deal if it takes twice as long to process canteloupe or your rib roast slices have some saw marks. Still I'm used to what I'm used to.
When cutting through a honeydew or a personal watermelon, usually I use two hands, one on each end of the blade, and a fair amount of pressure. The 8 inch puts my hand too near the sharp point for comfort. Also makes cutting a boneless rib or strip much easier.