I know that sm57 and sm58 is very similar in circuit design.
I already got a sm57 for instrument recording, just want to know is it worth to
buy a sm58 for recording vocal or double micing for my instrument recording.
If you already have a 57, I'd buy a Studio Projects B1 or C1 or something along those lines before going with a 58. You'll have a lot more versatility than buying a 58 when you already have a 57. I personally think 57 are much better for instrument micing, and then having a large diaphram Condensor like the B1 (cheaper) or C1 (a little more expensive, but great on vocals) will most likely be much better for vocal recording.
I know that sm57 and sm58 is very similar in circuit design.
I already got a sm57 for instrument recording, just want to know is it worth to buy a sm58 for recording vocal or double micing for my instrument recording.
Yes, you can't go wrong with either a SM58 or SM57. I would then save up for a moderately priced condensor like the AT4050, which will work on a wide variety of sources. After that, a pair of SDCs like the SM81s, which are great for home recording or live sound applications.
The SM57 and SM58 are similair, but I prefer the SM57 for both instrument micing and vocals. I agree with the others that suggest getting a condensor next.