It depends on what you want. The AKG C391B has a pretty flat response. I like them on guitar - most of my other SDCs are a lot brighter. I've never tried the bluelines on vocals - although I also use them for choral recording.

I have a couple of samples if you like - both guitar. Both have been mixed, so perhaps not a great reference for what the mics sound like. Both are stereo mic setups.

This was a quick studio session - the guy came in a recorded a full album's worth of material in 3 hours. The guitar was mic'd XY with cardioid blue lines.
http://www.cheap-tracks.com/mp3/kiser_guitar_sample.mp3

This one was a live recording of an acoustic show in a small bookstore. The mics were a couple feet from the performer - MS blue lines (CK91 & CK94) directed at the guitar and a Rode NT2000 overhead for vocals.
http://www.cheap-tracks.com/mp3/cheap-tracks_trina-lazy-river_sample.mp3

Thanks for that.

OOPS! - just noticed the second link was bad - put the file in the wrong place. That's fixed now.

Check out this thread below.

AKG C 391 B vs. Peluso CEMC6