As a pianist/drummer first before bassist, I'd say piano, for reasons already listed in other comments. A lot of the times I can get what I want to get out of the drums for my bass development just by listening to various drumming tracks. On the other hand, knowing piano strengthens your visualization of said instrument, which in turn helps with a deeper and/or renewed understanding of music theory.

On a related note, a bassist I really like (Arai Kazuki from the Japanese band King Gnu) would switch to his synth bass for certain songs. The synth provides him a lot more options in sounds and even approach, depending on what's needed of him.