I've never bothered trying to fly fish sitting in either, even with a 10' rod. Many rivers aren't that suited to a fly rod in my opinion. When you consider that most fish holding banks are a jungle for a fly rod, its not worth it on many rivers.
As far as the solo vs kayak argument, you don't give up a lot using either, granted there are some areas where one shines over the other, but when you consider everything, one will outshine the other for an individual.
I own both and while I probably use my solo most often, I can't say I'm ready to get rid of my kayaks.
No one has mentioned using either on a lake, probably because there is no comparison between a SOT and a solo, the SOT wins hands down.