Re: which ross evolution??
The reason is weight. That 3.5 may not even be heavy enough to balance your 8 or 9wt rod, but I can say for sure the #3 won't be. Having less total weight might seem like it would make it easier to use, but putting a light reel on a heavy rod will move the balance point forward and therefore greatly increase the "swing weight". I realize that it doesn't seem like a reduction in total weight would cause an increase in swing weight, but this is exactly what would happen.
So I would go with the 3.5 model if you're going to put it on a 8 or 9 weight rod. I would also put 20 lb backing on there if you're getting the Evo 3.5. The better suggestion would be to get a Ross Canyon Big Game #4. That reel has a drag that is more ideal for hard pulling big fish and also has more room for backing. I think its also the best sounding reel out there, but that's just me.