I need to decide weather to use javascript library or mobile sdk, but only one question will answer this.
Does google mobile SDK requires google maps to be installed on the mobile? or it just gets the map directly from there server like google js api?
No, the Google Maps app doesn't need to be installed on the device, in order for the mobile SDK to work. The mobile SDK will get the map data directly from Google's servers.
The only thing which does require the Google Maps app is the Google Maps URL Scheme, which is used when you specifically want to launch the Google Maps app.