Not all animals are square. Patagosaurus was a very primitive sauropod, so it's most likely that it didn't have such a long neck, or a high hip height.

Patagosaurus was still 4 meters longer, which I think is a pretty big difference.

That's just the tail.

They have equal body length when you subtract the neck and tail, and the tail seems like a major part of sauropod weaponry based on Diplodocus. Also, according to my size comparisons, Opisthocoelicaudia was about 2.5 meters tall at the shoulder and Patagosaurus was 3.5 meters tall at the shoulder.

But your size comparisons are outdated so... it's a 50/50 now if you ask me.

What do you think the shoulder height of both creatures was?

About 3 meters. For both.

Do you think that the bodies of both creatures were the same length as well?

I think Opisthocoelicaudia's body is almost a meter longer than Patagosaurus, although it's not that much of a size difference.

I also don't think that the body would be of much use in a fight.