I didn't use Horseman's Blade outside some testing, as I didn't fight the Pumpkin Moon yet (without testing via inventory editors), so I'd vote for Terra Blade, but... I think they're both good and I couldn't tell which one is better. They're both useful for different things. The Terra Blade is very useful for bosses due to its rapid projectiles. With extra gear rising your critical hit percentage/base damage, it can deal incredibly high damage. The projectile can easily deal about 200 damage and 300-400 critical damage. The Horseman's Blade is useful for events and invasions due to its homing pumpkin projectiles. They can hit multiple enemies flying around you and it's extremely useful for the Pumpkin and Frost Moon. Both are good in my opinion, there isn't any "better" in my opinion.