Thats a really tough question without knowing a general description of the situation. Is it a large fertile limestoner in high flows in the spring, a tiny infertile freestoner in low flows of mid-summer? It's tough to tell you a one size fits all situation. But, I'll do my best to describe my thoughts, just realize that these are guidelines, not rules, and yes, I do on occasion violate my own guidelines.
Small, infertile wild trout streams: Generally these are brookie streams, but not always. Anyway, I do dry flies 90% of the time in these situations. Fish are aggressive and hit just about anything, but often spooky. A dry allows you to fish from a greater distance, and float over obstructions and stuff. About the only time I go underneath is in wintertime (say, November through the first of May), or on those situations where you get that super deep pool where they're tough to bring up. I generally fish a size 12 or 14, and for the dries, something that floats high, as you won't get many backcasts to dry the fly, and something that is visible, that you can see in heavy current.
Larger, more fertile streams: Generally these are brown trout streams, but not always. In more fertile water, the fish tend to be pickier, and require putting it closer to them in the drift, they don't "chase" as much. I probably do 80% of my fishing with nymphs, sometimes indicatorless, sometimes with a dry-dropper or indicator. I go on top when the fish show on top. Fly size and pattern varies considerably in an attempt to match naturals.
Winter, or high-muddy water: For me, this is where streamers come into play.
