The longer rod would be better for smallies; but the biggest consideration if you are only going to get one rod (which is a good idea by the way when you are starting out in fly fishing) is to have something that will work better on the small streams you mentioned you will be fishing most with it. The longer 8'6"-10' rods would be better on the larger streams and rivers for smallies; but they will get hung up in the tree branche canopy a lot on the small streams in your area.
A shorter rod can always be used on bigger water; but a long rod cannot always be used on a small stream with a tree canopy like those in the Allegheny Mountains.
I strongly suspect the fish kill on the rivers was due to very high water temps due to the high air temps (it was in the mid-90's to low 100's each day we were there and lower than normal rainfall this past summer. My family and I visited my father the last week of July and the air temps were very high. Dad told us it had been like that since early June, so the river temps would have been very high with the low water and high air temps that had lasted for all those weeks.
Since you fished the Susquahanna, you know how large a river it is. It is also an excellent smallmouth fishery. Despite its size, I was able to fish it effectively with the Fenwick glass 7'6" 6 wt rod I bought when 15 and used through college.
As a point of reference, that Fenwick rod, one of the best glass rods on the market at the time, was $90.00 in 1968 when I bought it (in today's equivalent dollars it would be about $350.00) and it took a lot of hours working for $1.50/hr to get the $90.00 to buy it back in 1968. Now there are rods on the market for the same or even a little less that are as good as or better than that Fenwick and it takes fewer hours of work to be able to get the $90.00 together to get one. Newcomers to fly fishing are entering it at a time when there are some very nice rods on the market for rather low prices compared to 30 or more years ago.