AT series mics are usually generally bright from experience and comments i hear. My guess is it would be one of the betters for acoustic guitar, but not a strong point on voice. Although it does WORK on voice but its nothing special on it, but i have done some decent vocal recordings with the AT4033. Why not consider the SP C1, i have tried it on guitar and although not its not hyped in some places that make the
acoustic guitar shine, its really smooth and crisp, and better yet, will probably sound better on your vocals.
The SP B1, i think is the one with the tube in it isnt it? Im sure it is a nice warm sound that could sound good with your acoustic depending on what kind of sound you want, but being tube, its risky on its versatility. The coloration may or may not sound good on your voice.
Behringer - Boy if you have too

. Behringer makes decent home studio type equipment, but like mentioned below, a little too bright. Also, pay the extra money for durability even if you dont think youll need it, because you probably will.
A little story to go around with durability, and maybe a lesson for all of you youngen

. Last saturday i did live sound reinforcement for a park music festival. I had the whole setup there and things were going great. Turns out there were a couple of water bottles laying around by the mixer and stuff that were full of water (was a hot day). However, some dumb idiot (was probably me) didnt screw the cap on one of the bottles tightly. Next thing i know it, a half a bottle of water was spilt all over the mixer. The lights started flashing and and going hay wire but i turned it off in about 2 seconds. The team emmediately started cleaning the water off, unscrewed the back of the mixer to dry off parts inside. After about 10 minutes we tried her on again hoping for the best. Not a problem, the mixer started right up without a problem. We continued to make sure the water was cleared out of it but keeping the back plate open and the raising it up a bit to air out the water.
Two lessons:
Dont leave food or drink around expensive equipment.
and...thank the heavens the mixer was a mackie.
Dunno if a behringer or other type of board would have held up the same, but i do know that that was a lot of water, and the mackie held up strong and kept the show goin. Knowing this, i will keep my trust in mackie and spend the extra money for it everytime.
werd
Darnold