I always go with 27 point-buy. Because starting with 18, which is +4 bonus, is too much for a tier 1 character. In the other hand, rolling a low score like 5 or 6 would make a character non-playable. With 27 point-buy method, you can adjust whatever you want within a reasonable border. Also, monster stats are optimised for standard array, so if you go beyond the standard array, you start to bend the game.