Yea, the sm57 is a standard do everything mic, but isnt the best mic for the money. I have helped DCI groups as well as WGI indoor lines and we left standard dynamic mics alone and went to the condenser mics. They can cost more but a cheap solution for some groups have been the SAMSON mics. They make a cheap condenser mic set that has 2 mics in a box with shock mount. We also found that micing from the top provides a much truer sound. You do get the battery and horns thru the system but the pay off is huge if your after quality sound. You dont listen to a marimba from the bottom, why would you amplify resonator tone? Just a thought if you care to try. we mount boom arms on the frames of our yamaha keys and hang them over the keys. Check out Infinitypercussion.com for pics. We are an Independent world line pushing 4500 watts of sound.
The Samsons sell for about $100 for the pair. Alot ofo groups here in Florida are going to the RODE NT5 as well. More money, but a stronger mic in general. Just make sure your board has Phatom power and you will be set. You may want to research a 31 band EQ as well to help combat Feedback coming from the speakers. but once you start to hit feedback, you are close to the maximum gain you will be able to get with out some give/take on placement and volume.