It tastes pretty much the same. The only thing different is the manufacturer.
The taste is practically identical. Just the manufacturer is different.
「ほとんど同じ」＝ "pretty much the same," "basically the same," "practically identical"などなど
「が違うだけだよ」= "~is all that's different," "the only thing different is~," "just the ~ is different"
It tastes pretty much the same, it's just that it was made by different producers.
ほとんど pretty much
同じ the same
違うだけ it's just that ~ different