I do not know, if there is big difference. Generally, I use only suspend and go to off only, if I forgot to charge or because of firmware update etc.
I haven't paid much attention to how long it may stay in suspend, but it seems to me that it is several weeks.
Suspend mode has 2 advantages for me. The reader starts almost instantly and while in suspend nice pictures may be displayed.
This heavily overweights the disadvantage of
slightly higher energy consumption then power-off.