Spaceship designs, what seems practical?

So I've been pondering how spaceships would be designed. Now from what I have seen, they mostly go by naval or air force design, specifically for human factions and some aliens. But I read a lesser known author called H. Beam Piper, specifically his novel Space Vikings. His stories humanity used anti gravity and the feudal sword worlders had spherical ships and it made me wonder if that is a practical design.

What do you guys think?