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Internal volume is only relevant as part of a series of factors which contribute to overall BTU output at a certain delta-T, and with other factors being equal, more water volume actually = better CoP.
Material, form factor, size only matter in so far as the goal is to maximise water volume and emitter surface area, and minimise flow temperature and flow resistance on the circuit.
Talking of radiators, most fancy ones, especially the ubiquitous 'column'-style ones that mimic cast-iron style, have significantly lower real-world outputs than their stated BTU values, which are often just made up. Orthodox P+, K2, K3 panel rads are a known quantity with a uniform design having been through multiple test labs which reliably confirms their actual outputs.
Radiators at the end of the day are radiators, as long as you have the correct BTU for the room it’ll heat no matter what kinda of input.
Price wise they ain’t cheap tho