The hoop-style ones are stronger, more resistant to breakage/squash-age from wind, snow, predator climbing on top. The box-style rectangular ones are weak because there is zero diagonal bracing so all its strength against diagonal forces rests on the structural integrity of the itty bitty amount of plastic where the connector overlaps the pipe. I'm not saying you can't make the rectangular style ones, just recognize it's more vulnerable.
Also, be aware that normal PVC is not very UV-resistant and will weaken to the point of shattering easily within a year or two. There is some other kind you can get, I do not remember what it needs to be called, that is more durable. But *any* PVC run is going to be shorter-lived than the equivalent made out of metal or wood, and if it's supposed to keep out predators as well as keep chickens inside, you need to be careful as it ages, you don't want a *dog* to be the one who finds out that it's gotten real weak.
Good luck, have fun,
Pat