HVAC Heat Supply - Register type (Square Register or Round Diffuser?)
I'm in Toronto, and I'm a HVAC tech. I do use round diffusers from time to time but I've never noticed them being the "go to" by any tradesmen. There must be an undisclosed reason for it. Possibly a local supplier has been pushing them out at a low price. They (round diffusers) work well enough in most cases. Basements should be supplied low wall anyway but that often results in boxes and nobody likes boxes.

Also rectangular registers have an effect called throw. Depending on the orientation of boot (A boot, end boot or universal boot) the outgoing air will have momentum and upon hitting the vaines of a rectangular register deflect but continue slightly in the starting direction.

rectangular register with an end boot



continental industries

Round diffusers have different effects illustrated by the two following images. 
Round diffusers



researchgate.net



Slideshare

Honestly the dynamics involved are chaotic and almost impossible to predict accurately. I studied this topic deeply years ago. It's something I still consider when designing a system but ultimately customer want to eliminate boxes first. If given the choice, not so mechanically inclined customers will always go the "no box route" and customers who take great interest in the design of the system may be okay with a box. 

Back to the OPs question round diffusers may or may not work as well as a rectangular register. For cooling they are definitely better, for heating, in my experience at least, they don't work as well. But in a basement where half of the exterior is underground heat loss is reduced compared to the floors above when it's exceptionally cold outside. 

In short there is no clear answer but at least there're taking low wall returns.