Not sure what your question is. Pick whatever appeals to you--the third and fourth times you learn a new key, it gets much easier. You're extremely fortunate to have access to instruments of all four keys.
Why? Are you looking for something with a different sound from what you are playing? Do you just want to learn to play a different horn for some reason?
I think that if you already play BBb, then an F or Eb would be good to add to give you the ability to use one with small groups. Which you pick really doesn't make that much difference, since you have both available. If you think that you may buy one in the future, then Eb's are less expensive than F's in general and so that might make it a better choice.
How old are you and what kind of playing do you do? If you are in high school and contemplating a college tuba "career," it might be a good time to learn CC. If you are an amateur and doing it for the fun of it, I would go with an Eb.
mandrake wrote:A wise person (me) once said (just now) to keep the flats and naturals separate. Why? I don't really know that there is a good reason, but it makes sense to avoid tubas which are pitched a minor third or a tritone apart when you can take a Perfect Fourth.
Um... Unless you're talking learning both CCb and F, or BBb and E, or CC and Gb, or maybe GG contra and the DDb tubas everyone's been talking about, I'm not really sure how you'd get a tritone.