Hi All

Am going to be recording singer/songwriter friend indoors. He'll be playing acoustic guitar.

Am planning to use a Behringer Xenyx mixer, fed with a Rode NT3 for vocal mic and i was thinking a Rode NT5 as the intrument mic for the guitar.
But now, after trawling numerous threads, am considering the AT3031 (also a cardioid studio condenser like the NT5) instead of the NT5 for the guitar track.

Would anyone like to jump in with their opinion/view on which of the NT5 or AT3031 to go with for the instrument mic?

Also i've heard the NT3 is super-sensitive to air-movement.
Opinion on whether i'll need to use a pop-filter if the NT3 is about a 12-18inches from singers mouth?

ps. (Rest of the stuff .....I'll probably be taking the mix off the Behringer desk and recording to my iRiver HDD recorder as hi-bitrate MP3 or uncompressed WAV.)

All the mics you are considering seem to be good choices for acoustic music. One thing to consider is the polar pattern of the mics you choose. When you have open mics that close together phase issues can really be a problem so I would make sure you are using mics with a tighter pattern.
I would always use a pop filter. I have been running a recording studio for 10 years now and have only gone without a pop filter a couple of times, better safe than sorry. You mentioned the vocal mic would be 12 - 18 inches from the singers mouth. I would go as close as the singer would be comfortable with to eliminate as much bleed as possible from the acoustic guitar. I shoot for about 6-8 inches usually.
I use a KSM32 for vocals and an AT4033 for acoustic on this type of job. My thinking is that you want the more accurate mic on the guitar and the more colorful mic on the vocal.
Another real important thing to consider is how high the ceiling is. Beware of low ceilings as they will cause comb filtering and sound hollow no matter how good your mics are.
I hope I did not insult your intelligence with stuff you already knew!
Good luck
Mark
Oops! I just realized I was on a video forum, NOT a recording forum. Having a big ol' studio condenser mic with a pop filter a few inches from the singers mouth may make for lousy video. If presentation is important (ie actually seeing the singer) you may want to go with a Beta 58 on the vocal. They still sound good, but won't get in the way of the video.