Short answer: Berat is easier from Tirana and prettier by the river; Gjirokaster is more dramatic, stone-built and museum-rich. Visit both if possible.
Short answer: taxi or private transfer is easiest from Tirana Airport to the city center; the airport bus is cheapest; rental car is best only if you leave Tirana quickly.
Short answer: September is one of the best months to visit Albania. Warm sea, fewer crowds, better prices and ideal conditions for Riviera beaches, Berat, Bovilla and Komani Lake.