Which android phone to buy ?

[dflat]

1) I play mainly the double bass (if someone here plays it to with ireal pro ), and a little piano to work.

A consideration is if you stand upright while playing bass, where the music stand/device might be nearer to you or if you use a stool with the bass sloping against your shoulder, where the music is perhaps further away from you; and also finishing playing, if the music stand will then need to be shifted away from you in order to put your bass down.
Although you can put your device on a music stand, because they are smaller, there are dedicated holders for different sized devices that can attach to microphone stands (etc.) and many musicians use these instead. This could be another idea to think about.

5) Ok, so what is the minimum screen size on a phone to be confortable reading chords while rehearsing ?

Attached is a .png to print out. It is a typically full chart so the chords are quite small. You can print it out using the Scale setting in the print window from your computer so that it is 4 inches (or whatever) (diagonally) across. This will approximate a small screen and you can move it around on your music stand. It comes from a small screen iPod touch, so it might blur a little when you scale to a 10 inch tablet screen size, but this is only because it comes from a small device.

7) For some reasons i prefer Androir over iOS, but it's true that it seems to be better apps on iOS
I was considering maybe a cheap but not too bad chinese phone like Huawei or Haier.

Yes make sure you have good reasons for considering Android (or iOS). As I suggested, look at the apps available as these are what you will be wanting to use. The OS is secondary in my opinion.
iOS for example has recording apps (4 or 8 ... and up to 48 tracks) depending on the app (not many at all on Android.) Apple's Garageband is very cheap and there is nothing like it on Android. Both have PDF apps (iOS has quite a few more to choose from), used for your notation, PDFs, lyrics etc. iOS has midi (Sampletank for instance), synths and other odd music creating apps that can be lots of fun to play with. Also if you teach, consider apps related to that. Both have tuners. There is a nice selection of metronome apps on iOS.
Both Sibelius and Finale have iPad apps although both currently are very basic. I expect them to add features in the future because more musicians are using iPads for rehearsal and performance. Notion and Symphony Pro (both iOS, not Android) can be used to input notation (although computer software like Sibelius is much faster).
iOS also has a number of excellent guitar apps (for anyone else reading this thread) like Amplitube (which has pedal apps and recording).
And as you know, iReal Pro runs on both.