Here's my and it's probably wrong.
1. Some people just like having the best or as close to the best as possible. More power to em.
2. In terms of actual condition between 9.6 and 9.8 I think it's in the eye of the beholder. I would guess experienced collectors would tell you they've seen 9.6 books that look like 9.8's and 9.8 books that look like a 9.6.
No matter how much of a "collector" you are, it's hard to buy any of these books without thinking of the resale value.
Keeping that in mind 9.8's have proven to bring many multiple of guide compared to their 9.6 counterparts, despite the sometimes negligible condition of the actual book. So when you buy, if you tend to lean more towards the buy then sell side of the hobby, I would go with buying the 9.8's.
Unless of course you are into the crack/press/re-sub thing, in that case buying 9.6's and turning them into 9.8s could be more profitable.