I have one of each and the only possibe advice I can think to give you is - fretless 8 string bass. The jangly tone of the 8 combined with the punchy mids and sloppy intonation of the fretless is just a pure dream come true.
All joking aside - I assume you have a fretted 4 now, so I'm happy at least someone in this forum is thinking about buying a genuinely different 2nd instrument & not "well I have a P, should I get a J?"
An 8 can sound sort of sinister (a lot of later John Paul Jones stuff is with an 8), especially in Jazz configuration & takes distortion well - though if you have two guitarists in your band you've basically taken over the role of bass AND rhythm guitar. It's also more difficult to make high speed runs, due to the extra tension the added string makes (though a 32" or 30" scale helps - if you can find one with this scale length).
A fretless requires more finess & accuracy on your part, but it would be a good way to develop your ear. A fretless also draws less attention than an 8, as fretless is just sort of a natural variant of the bass, the 8 is a bit of an odd beast.