vertical radiators

If I change a conventional (horizontal) radiator for a tall vertical one of the same BTU / Wattage, will there be any differences in the heating effect?

Not really, but the heated air won't follow the same path which may be awkward. Because they're taller and narrower, the patch of ceiling above the radiator will be smaller and hotter which may or may not matter.

I have a couple of 3kW verticals - big buggers - in the kitchen and hallway and they both work well, but kitchen one is higher output than the conventional radiator it replaced.

They're also a lot more expensive and heavier, but you do regain a lot of dead wall. They look better IMO too.

Cheers

tchen one is higher output than

I have one about to be fitted in a narrow space in my bathroom - like the look of it so much I was thinking of changing a couple in the lounge - as you say - you regain some wall

Most of the heat input will be delivered directly to a thin band of very hot air close to the ceiling which will eventually fill the room.

Well they do actually radiate some heat too. More so if they are 'modern' and black.

To look effective they need to be installed with hidden pipework, so replacing an existing rad is not straightforward.

They radiate a only tiny amount and any paint colour apart from polished metal is "black" in the thermal IR band. Most of the heat will be convective and make the ceiling a really nice place to levitate too.