I'd say it depends on what else you already got going on on your board, the type of sounds you're going for and what context you want to use it in.
I think having a delay leads to some more creative stuff musically wise than adding tremolo. I love me a good tremolo on bass though, it adds a feel I really enjoy more than chorus.
The Dr Scientist Tremolessence, the EQD Hummingbird and the Empress Tremolo were some of my favorite tremolos.
For delays I think choices are a bit more complicated because there are so many differents things a delay can do and it really depends on what you're going to do with it to decide which one would be good for you.
If I add to choose only one I'd go with a delay. But you should buy a bigger board and buy both.