Live stage recording and live concert capture are not necessarily the same thing. On stage mics are much better off in the dynamic moving coil variety. For acoustic concert capture the small diaphragm condenser microphones are the preferred version until you can afford a great stereo ribbon or ribbon pair.

If you want good AT ambient mic's then the AT4051 are king with it's cousin the AT4041 close second. The Oktava are hit and miss unless you are purchasing modified versions ala Michael Joly or similar. There are MANY counterfeit (or were) Oktava mic's on ebait and various other online sources once upon a time.

thanks for you answer.

I didn't know the those AT mics, i'll have a look into them.

Anyway I have to say that my plan is to use the mic in the room, not really next to the instrument, that's why I was planning to have the possibility to use an omni mic: for this reason i was considering the mk012 and the akg 420, because they have an omni switch or capsule.

The Sennheiser me62 could be an option too, what do you think?

I would not choose an omni for your application. I do use omni pairs for classical recording but the room has to be just right for them to work well. In any sort of amplified environment the omni is not the pattern of choice with the possible exception of lavalier mic's placed on talking heads. Dynamic or condenser mic's with a cardioid or hyper cardioid pattern are much more useful in live environments.

Thank you for your helpful comments.

I finally bought an me 66, very directional and very sensitive.
I've also bought a me 62, for recording the ambient.
I have found both on ebay for a good price (around 200€ each).
Very happy with this choice