I would say go with the SM57. Kung pang-demo-demo lang, ayos na ayos na iyon sa vocals, lalo na't sabi mo nga maiingay ang kwarto - less ang sensitivity nito kesa sa kahit anong condenser. That way too, kapag nangati ka na mag-"upgrade" later on with a large-diaphragm condenser, meron ka pa ring "class" na dynamic na pinanghahawakan - wala ka nang "budget" condenser na kailangan pang ibenta.
Another alternative, and something that's like middle ground for the two mics, would be the Audio-Technica ATM-33a. It's an electret condenser with a pretty warm sound, sensitive enough and can handle high SPLs to be used for overheads sa drums (haven't done that, but some people have had good results), something that you can stick in front of a guitar amp (i've done that - qute honky, that i may like it better than large condensers for similar applications), would be sensitive enough for piano or even acoustic bass (i've mic'ed a guitarron with it - not bad), and more than anything will give you a very warm tone and good sensitivity for a vocal mic. Plus, if your mixer has no phantom power, you can use an energizer AA battery on it that'll last you for ages. Although bawas ng kaunti ang dynamic range if you go battery - it's best to phantom it always.
Sadly, they don't make em anymore. Although i've seen it listed at Sulit.com, for 5k:
http://www.sulit.com.ph/index.php/view+classifieds/id/2229332/For+Sale+Audio-Technica+ATM-33ANot a bad price, because i got a pair brand new some 7 years ago at around 7k or 7500, if i'm not mistaken. I use it for handheld location hosting recordings for video shoots, or as podium mics for narrative video/film shoots. Ayus pang kausap yang seller na iyan, si Dez, i know her personally from Audiophile years ago - she was the one who must've sold me my pair.
Another alternative would be a PG81 - that, i've used as overheads from drums, kaya nga napabili ako ng ATM33a, because it's a similar design - electret condenser na de-baterya nga. I've seen them listed sa Pro Audio Classifieds on occasion, around the same price or maybe slightly less than the ATM33a. Though I would say I like the Audio-Technica design better - i like the "ka-bruskohan" of this particular mic, while having enough sensitivity - and generally, Audio-Technica are bang-for-buck microphones, I trust in their design.
But if you're asking between SM-57 & AT-2020, i say SM57. But do try the ATM-33a, PG-81, or any similar electret condenser, like maybe the SM87 - they might be suitable for your needs and you might like the overall sound better too.
