[Amps] Bias or Plate voltage change

[GGLL]

	I've seen many GG amplifiers with a lower working plate voltage  for CW mode
than for SSB (for example some 2KV for CW, 3 KV for SSB).
	Others have a fixed plate voltage value and change cathode bias instead (7.5V
for SSB, 27V for CW).
	Why this? and which method works better?. The latter seems to be cheaper, but
is that the only reason?. One can work SSB with CW plate settings (not cathode
bias CW settings), but what about CW at SSB plate settings?. Too many
questions!, sorry.

Thanks in advance
Best regards
Guillermo - LU8EYW.