For same BTU is vertical or horizontal rad better?

[landlord]

Can anyone recommend going for vertical or horizontal radiators assuming the same BTU of each?

 

[theplumber]

Physics would seem to dictate that a long one will raise the whole room temperature more evenly and quicker.
As long as it is fully pumped circulation.
Vertical rads assist gravity water circulation more.

I am sure that a tall radiator salesman will prove me wrong.?