Objectively, a straight headjoint offers a better sound and intonation than a curved headjoint. Curved headjoints don't have the same ability to taper consistently, so they can make more notes hard to play in tune. But the best headjoint for you is one that's comfortable. Some people find the curved headjoint makes it easier to reach the keys. I have had a hard time balancing a curved headjoint alto flute when I've played them before.