Whenever I decide something has to go (financial reasons or to clear space, so to speak), here's the reasoning I use:
Which bass in my arsenal would cause me the LEAST pain to sell off. That's the one I'll put up for sale. But even that process is problematic after awhile because you get to a point where you're happy with all of them. But if you're honest with yourself, you can always choose one that you like a little bit less than the others...
Oh yeah, and don't beat yourself up after the fact. As long as you've given it some thought before you sell it, and had a good reason for selling, don't dwell on it. You did what you felt you had to do.