Interesting thread this one.
How about sitting in a boat in the harbour, 3500 acres of water in front of you and having to make the choice between going for the stockies (Rainbows) or going for the native brown trout in the same place!!
Of course i'm talking about Loch Leven and this was a scenario I was faced with many times when I left the harbour and at one time that could be almost every day.
This topic could have 1000's of answers. The key aspects is to "try" and master is the conditions. Temp, light, wind, air pressure, wind direction, time of year, water temperature, where the fish are and at what depth they are feeding, what the average size is and this is only the start!!
It was amazing how many times I thought to myself, right, i'm going for those rising browns we caught last night and during the day session we caught a bag of "Stockies" and not a Brownie to be seen.
This confirms my point, there are 1000's if not 1,000,000's of combinations to adapt and be successful but more often than not the same tactics will take both species. As far as i'm concerned, that is the fun of flyfishing.
Paul