Yo Frere Rat! OK. When selecting
acoustic guitar strings (I'm going to assume steel- nylon is a whole different ball game), the first thing that you have to remember is that guitarists are worse than a bunch of engineers trying to agree on a mic. Ask 10 different superstar guitarists, and you'll get 10 answers. In the end, you'll have to find strings that agree with *you*, but I can give you the options.
Once upon a time, folks used steel wound with nickel, but those days are gone. Pretty much, strings today are made of steel, with the trebles often being silvered, wound with bronze. Understand this right off the bat- Steel
acoustic guitar strings need to be changed *much* more frequently than your bass strings. Don't wait. 20 hours of play if you play Renaissance music, 6 hours if you play The Who. Get over it. Next, they will go dead much more rapidly than bass strings. Even if they don't break, the sound will be long gone.
That said, you need to select the type of bronze, the gauge, and decide if you want them coated or not. The 2 main types of bronze are phosphor bronze (copper and tin) and 80/20 bronze (copper and zinc) Phosphor bronze lasts longer, and is darker. 80/20 is bright, and begins to lose that brightness pretty quick, perhaps in 10-12 hours of play. Of course, there are supporters of both types. 80/20 is good for cutting through a mix, and I like phosphor on a big stage guitar. Probably more guitarists prefer phosphor, which also has excellent sustain.
Gauge- most string sets are sorted into light, medium, and heavy gauge, and you may see medium-light
or medium-heavy. The last 2 hybrids usually mix and match, with combinations of lighter trebles and heavier bass strings. Most folks use lights or mediums. Here's where you have to pay attention to the guitar. For every guitar, there is the gauge it likes, the gauge it is set up for, and the gauge it can survive. Bluegrass players like the higher action you prefer, and heavier strings, at least medium. This is loud, has superior sustain, and is good for flat picking. Not so good for bending strings. Heavies are rarely used, except on specialty guitars, such as baritones, a resonator/Dobro, or lap steel. There are a few bluegrass diehards that use them. Heavier strings place more stress on the neck, truss rod, and neck joint. Some older or more delicate guitars can just snap. This can happen with any string, but using heavies on an old guitar is rolling dice.
When a guitar is set up, it is set up for whatever strings the luthier or company intended to use, so note the gauge of the provided strings on a new guitar, or any used one you get from someone with a clue. The nut is cut with slots to fit the intended strings, If you are using mediums, and the nut is sticking when you're trying to tune, that's a clue that the guitar was set up for lights. That can be changed by a guitar tech, but changing it back may be a bitch. Also, the truss rod is adjusted to provide the right amount of relief for a given amount of tension, and changing gauge, or sometimes even brands, calls for a truss rod adjustment. My guess is you will prefer medium phosphor bronze, and a guitar set up bluegrass-style. Try out a brand new higher end Martin dreadnought, like a D-28. They are pretty much set up that way out of the box, and many people bitch about the high action on the factory setups. You'll probably like it.
OK- to use coated strings or not? Elixir really started it off with their polyweb strings. They are coated with a polymer, basically plastic, which is almost invisible. This is intended to protect the string from corrosive acids in your skin oils and sweat. It does make the strings last 2 to 2 1/2 times longer, and gives a more gradual, linear loss of tone toward the end of their lives. The coating also reduces finger squeaking when moving up and down the neck, and are a little easier on your fingers. Note that not everybody's sweat was created equal, and if you are a person who sweats a lot, and your sweat is acidic, you will get a bigger benefit from the corrosion resistance. As far as the sound goes. the crowd is clearly divided. Some very knowledgable people will tell you coated strings are the work of the Devil, and kill your high end detail. On the other hand, many highly skilled acoustic guitarists use them. Only you can decide what sound you like.
Some definite brands to try are Martin Marquis (not coated) D'Addario (not coated, unless they have started making coated strings), Elixir Nanoweb (same as polyweb, but thinner coating- theoretically less change in sound), and Cleartone (a different formulation of coating- the new kid on the block).
Hope that helps-Richie