I think that if you are planning on going at least 9.5', you might as well go for the full 10'. The extra length can make a world of difference when mending line, getting the extra distance casting or even high stick nymphing. Is there a reason that you are not looking at two handed rods?
I have found 10' footers to be a little tiring when casting all day or on multi-day trips. I'm sure there are some 10' footers that cast better than others and so on. Surprising 6" can make a difference with mending and roll casting. So many choices? Think of the water you will visit the most and what rod length would benefit you the best and that should help with the descision process. Good Luck!
I haven't used a 10' except for a few practice casts, so I can't really say much about that length. I had a 10 1/2' for a few years, but the extra length didn't seem like an advantage, and it was more cumbersome to carry, both on the river and in the car. I like a 9 or 9 1/2' rod in an 8 wt, and have had a 9 1/2 for the last 14 years that would see more use but for the collection of Spey rods that keep it gathering dust.
You ought to try both lengths and see if one sings to you, or if the length even matters. Have fun; you won't likely go wrong with either.
It's not how big the rod is, it's how you use it.
I got my first 10' 8wt for bone fish.
When I took it Steel heading I found it to be heavy, lots more casting.
I found another 10' rod on sale that weighted a few oz's less and it has been a dream to cast.
Check rod weights while shopping.
Best thing to do is take out a mortgage on your home and buy 10 rods....that is AT LEAST 10 by the way. Just saves you a lot of time wasted making decisions like this and gives you more time on the water. In 10 years you will have 10 rods anyway!!!
Unfortunate timing - I was looking for an old GLX 8wt, but finally gave up and bought something else recently. I love those rods - have a 5wt and 6wt old GLX. I wish they still made them in something other than 2 piece models.
I mostly use a 9 - 9.5 for all my steelheading. Mostly because those were the lengths they had available when I bought them. LOL. I grew up fishing an 8 1/2 glass rod for steelhead. Had no troubles.
Personally, I feel all new casters are required to use a glass rod. Fish them all day. Put an old clunker pflueger on it as well. Then go and get yourself a graphite. You won't complain about a few ounces all day. LOL