Which oral anti biotic is better for acne doxycycline or minocylin ?

No difference: Doxycycline and minocycline are almost the same antibiotic, only a minor chemical difference -- equally active against acne (or any other infection). Mino has generally been used more frequently for acne, but that doesn't mean doxy doesn't work well. The main difference is that doxy is more likely to cause photosensitivity, i.e. tendency for sunburn. Mino might be better in sunny Portugal!