Small budget-wise sm57 definately,... I've worked with engineers that have actually went into the studio and yanked expensive instrument mics in favor of the ALWAYS reliable clean sound of an sm57. Of course, you can always upgrade, so the 57 is a GREAT choice... and, you need to have on or two layin' around anyway !!! Hope I helped.
These two mics are completely different animals altogether. The Apex is a large diaphram condensor, overly bright (in my not so humble opinion) cardiod mic. The 57 on the other hand is a dynamic work horse. What kind of instruments? Pipe organs? Kazoos?
I've never heard a B1 so I can't say. As far as placement goes, experiment until you like what you hear. With a 57, conventional wisdom is to put it tight on the amp. If it's aimed at the center of the cone, it'll be more strident, towards the speaker surround, the sound will be more rounded. PLay with it till you like it.
Another option in the same price range as the 57 is the Sennheiser e609, which is supposed to be tailored for micing guitar cabinets. I've never used one myself, but maybe someone here has. Looks like you can get the e609 from http://www.northernsound.net/Sales/salesframe.html for about $90.