I believe, and correct me if I am wrong as I probably am, that the steelies in the great lakes function differently than the NorthWest ocean going steelies.
If I remember correctly, the great lake steelies head to the great lakes, as if they were the ocean, and then back to the streams/rivers/creeks to spawn.
Meanwhile the NW steelies go to the ocean and return to the streams/rivers/creeks to spawn. (Which I am sure everyone already knows)
The fact that the NW steelies go out to the ocean is what helps to make them bigger. The ocean supplies a completely different ecosystem than the great lakes can. Apart from the fact that it is salt water system and holds a much larger variety of prey, the salt water itself affects the steelie's body in a completely different way on a biological level than does fresh water.
So, as far as a difference in size and why, I would say that it is due to the lack of saltwater in the Great Lakes Steelies.
As far as which rod to use, I couldn't really give you too much advice about that but I would really want to use at least a 7w. But that is me personally.
a 5 wt is a trout rod.... but if you want to just watch one fish after another disapear... then a 5 is your horse, small creeks require being able to stop or turn fish, any steelhead over 8 lbs is gonna punish you and leave you dissapointed-
I would definitely go with 8wt I have a 6/7 weight I have caugh some 18" trout on and I dont think it would do trying to stop a fish going down a riffle especially during the winter I have a 9/10 that I have caught steelhead and salmon on and it works well I also have a 8 that i have only caught one coho on. If you were thinking about tarketing chinook also I would go with a 9/10 weight. Also a good reel with a drag just not a cliker or your hands will be beat to death I have a loomis reel on my9/10 and a okuma on my 8 and I like the loomis better.
Thanks for all the info, looks like I'll be gearing up a bit with my spinning/casting outfits. However, If you ever see a guy frantically running through a stream that's me not taking sound advice and going with the 5# in spite of all these great tips.