a nine foot, five weight graphite is the most popular trout rod and is very versatile.
I have one but I also have a nine foot, seven weight that I originally bought for steel head and it is also very versatile.
The biggest difference, comparing the same model, is the rod weight. A seven weight is going to be a little more work to fish but not that much. What you get is a rod that can handle larger/heavier flies as well as larger fish.
My seven weight is a discontinued Winston BIIX and it is as light as my older five weight. Since then, other rod makers have also come out with lighter rods. I have nymph fished with that rod for 8 or 9 hour days with no complaints.
I even used it once on a windy day for dry fly fishing and it worked fine. Out west, the lighter seven weights are supposed to be more popular because the folks out there contend with more wind and fish with more weight, in general.
Just my experience. It really depends on what you want to do with it and how you fish.
Shock