Also, if you change to a different gauge of strings than those already on your guitar, you're probably going to have to do some setup work. In this case, you'll need to take it to a tech and have them make the necessary adjustments to accomodate the different tension put on the neck by the different gauge strings.
Lighter gauge strings are under less tension when tuned to pitch and are easier to play on. They'll play closer to the way that an electric guitar will play. In my experience, lighter gauge strings will yield less volume and a less focused, flabby kind of tone, especially when played or strummed hard. I'd stay away from "extra light" gauge acoustic strings. Light gauge is as light as I would go. Most companies give the "light" designation to their .012 gauge strings.
My personal preference for gauge is Medium. Most companies give their "medium" designation to their .013 gauge strings (although there are a ton of different combinations/permutations of the gauges of the other 5 strings in the set). I find that medium gauge strings yield more volume and they sound very solid in the lower registers compared to lighter strings. An open E or A string really sounds tighter and more focused to me when compared to an open E or A string from a light gauge set of strings. And they still sound strong and focused when played/strummed hard.
Anything heavier than a set of .013 strings really puts a lot of stress on the neck, bridge and top of the guitar. I'm sure that a lot of guitars can handle the stress of a set of .014s, but I can only imagine that a lot of guitars would lose a bridge or have a deformed top due to the insane amount of tension produced by tuning a set of .014 strings to pitch.
As far as materials, I've always been a fan of Phosphor/Bronze. Not really because of an informed opinion, it's just what I've always used and thus I've grown accustomed to their sound.
Ernie Ball Earthwoods were what I used for the longest time. Well-priced, they sound nice and brassy when they're new, although they go dead pretty quickly. I actually prefer the sound of Phosphor/Bronze strings when they've died slightly and lost some of that sharp, brassy tone. But when they go totally dead, yuck, they're the definition of "dead" sounding.
When I purchased my new Martin recently, they sent me 13 sets of strings for free. They're all medium gauge (.013 - .056) Phosphor/Bronze. I do like the way they sound, although they're on a guitar that cost 10 times what my other acoustic cost. So I can imagine that the guitar itself makes most of the difference, and the difference between Ernie Ball and Martin strings is a pretty small concern in the overall tone.
Get a good string winder and an assortment of strings and see if you can hone in on the ones that you like best.