Both are not regularly available in my country
Maybe I can try to buy second hand asrock, but biostar is totally out of my range if I don't want to pay a lot for shipping from USA or waiting long because of shipping from China.
So this is where you figure out what
is available in your country for a price you are OK with, and build up from there. Will it be The Best™, maybe not, but maybe for your country and your limits, it will be.
Since you want to scale up, maybe try building a four GPU setup first, and get that stable, and then build a bigger rig. You are also likely going to want/need powered risers, so don't worry if the slots are X1 or X16, unless you are getting X16 to X16 risers.
So start with which motherboards are available to you and go from there.