I started singing in a choir last year, but I still haven't figured out whether I'm a soprano or mezzo-soprano.

When people (or websites) state that the soprano range is from G3 to C6 ... is this the "power range", the range that sounds good? Or is it the complete range of the voice?

When I do warm up exercises, my range is pretty much the same as stated above (G3 to C6). But when I sing in a choir, I feel most comfortable singing between G4 and F#5. I struggle with the notes from G5 and above when I'm singing with the choir ... And even if I do manage to sing those high notes, it doesn't sound good at all.

Even so, two different people (my director and a voice teacher I saw just a few times last year) have told me that they think I'm a soprano. But I'm still not sure. I have a good range during warm up and vocal exercises, but then when we're actually singing I struggle with the high notes. Does this mean that I'm really a mezzo? Or would it be possible to develop my voice further, by taking voice lessons... could I become a better singer in the high register, or would I just be pushing to become something that I'm not?

Sorry the long post. I'm just confused, and would appreciate any input!

Your vocal type is better determined by your low notes. Does your low G feel comfortable and sound firm? Can you go lower with effort? If so then you are a mezzo. If your low G is breathy and weak and is the lowest you can go(except with a cold) then you are a soprano. Since you mention feeling comfortable as high as F5#, I suspect you are a soprano. It seems your confusion is caused by the trouble you are having wth your high notes(or vice versa?). It is normal for any vocal type to have trouble in the highest parts of their range, also the range you mention(G3 to C6) is for trained opera singers. Many sopranos in church choirs cannot sing higher than G5. Vocal training(the right way) can most certainly improve your range. I recommend Seth Riggs' book "Singing for the Stars" or his Singer's Advantage program. It did wonders for my voice, I am a baritone and can now sing smoothly up to C5 or higher. Good luck! I hope this helps.