Welcome on GS!
First thing to mention since I guess that is for your vocals and guitar.. Is that it depends..
What I mean is that for every vocal (or source) there's the right mic..
So the best way to go is to contact a dealer (or a studio near you) and test some mics out in your price range.. IME there's no other way to find out the best mic for YOUR vocals (in this case in your price range).. btw it's not forcely a matter of price.. sometimes with some vocals a 300$ SM7 (or a 1k Coles 4038) will suit better than a 10k U47, it really depends on the vocalist and the genre/what you want to reach sonically.
The Duet is decent interface and more than capable of recording a well written song, the preamps are clean but do sound good all considered..of course having a pair of dedicated preamps might enhance your recordings, but I'd go with the right mics first..(even if some mics behave differently with different preamps..like with the BAE DMP or the Avedis MA5).. I might just be concerned to have an interface that works only with the macs.. (RME babyface works on both mac and pc)..but that's me and let's go back to the topic..
Within you budget I'd try to test out:
- Violet Design Black Knight (serious mic for half your budget!)
- Milab DC-96B (it's a little mic that is stellar on ac gtr and sounds great on vocals..don't be fooled by its size)..below 1k.
- old AKG 414.. workhorse that might fit your vocals around 1k (I wouldn't buy the new ones).
- Coles 4038, it's ribbon mic figure of 8 pattern, very thick and mellow, might fit or not your vocals..it's not a workhorse for vocals, more hit or miss IMO (I tracked vocals with it and an MA5 with great success on a reggae singer btw), could be nice on some ac gtrs as well.. great mic for a little bit more than 1k. (this mic is a must have, but in your situation I'll choose the "right" mic first).
- AEA R84, somewhat "cleaner" than the coles.. Within your budget or slightly less.
- Royer 121, also a ribbon fig of 8, "cleaner" than the other two, great on el gtr, possibly also on ac gtr, I'm not so sure about vocals though (never used it on vocals).. But might be a good fit for yours..try it out.
- JZ vintage11, it's new mic from JZ that looks promising as a workhorse. Below 1k..
- Brauner Classic, great mic, very detailed, very open and airy, sparkling top end, might get too boomy on some ac gtrs if placed too close, great overall mic, suits some particular vocals I never thought of very well.. above 1k, but might be worthed or check on the used market, they go for about 1k.
- Violet Design Globe Vintage is a bit less than 2k and it's a helluva mic, it's like tracking with butter (you could read my review in the GS review section, below my avatar you have a direct link to these).
One or more of these mics might fit perfectly your voice (and/or ac gtr), others might be just ok.. all of them are great contenders btw..
Remember that the other very important factor when tracking vocals or ac instruments is the role played by the room.. If the room is shitty, no matter how good your mic, preamps., etc.. It will sound shitty.. Treat your room if it needs to..
Just my 0.02$,
I hope this helps you somehow..