I have received two different opinions on where to place cold air returns from HVAC companies.

I am developing my basement. My hot air supply vents are located on the ceiling at the outside walls by the windows. My cold air return intakes are located on the ceiling closer near the middle of the house.

One heating company said that it's fine to keep the cold air returns in the ceiling, that it will make for better overall air circulation then if they were at floor level. He basically said either have the heat supply and cold returns both on the ceiling or both at floor level, that having one on ceiling and one at floor will make for inadequate circulation.

Another heating company said that if I keep the hot air supply in ceiling that I should have my cold air returns placed at floor level. His reason is that hot air rises and the cold air returns at the floor level will help draw some of the heat down.

The most ideal I believe would be to have both at floor level but that would be costly and because this is a bi-level house, I would then need to frame all my outside walls to cover the heat venting pipes.

I always understood that heat rises and cold falls. So would it not make sense to have the cold returns near floor?

Any other opinions would be appreciated. By the way, I live in Canada where it can get to -20 to -35 celsius during January & February.