Both!! Being able to play a fretless is a very cool thing indeed, especially if you can actually play it in tune. But the sound of a fretless is distinctly different from a fretted bass and it's far better to use the right tool for the job. Some tunes just won't sound right on a fretless... some will sound far better.
If you are blessed with owning both, by all means, practice both!!! Use your sense of musical taste to tell you which to use when. Do a lot of listening and that will help you make those decisions.

Definately stick with both.

Also, if you can play the acoustic double-bass (fretless) in a string orchestra at your school (learn how to use the bow) that would help a lot.

I picked up the bass last year, playing it once a week for one school year (joined the ensemble after 1/2 year of private study) and ever since then I've improved drastically not only in bass playing in general, but in all my other music courses because playing fretless allows you to get your ears in tune. It's a great way to ear train using the fretless.

I'd stick with both but I like frets. Just my opinion...

If you can play the fretless do it! You can get a better sliddy slippin up the neck sound! Its not for everybody though,its harder to gage where the solid notes are.So if you can keep at it

frets are more hard rock

personally ive never really had time to play fretted bass, because one...i dont have access to one, and ive been playing double bass for almost 4 years. You should try double bass. It introduces you to the many ways you can play bass and trains your ear for fine tuning. As for a decision...play both! Why would you settle for just one when you have the freedom to do both??

Just like everybody else. BOTH! Of course. I have both and sometimes I practice a heavy metal song on my fretless. It does sound a bit smoother but I can play Killing In The Name by Rage Against The Machine on both and I think it sounds cooler on a fretless. Stick with both. Also, if you get good at fretless, you can start a double bass. That's really fun. Playing really fast bass lines on a double. It's a little different, I mean the strings are farther apart but it still rocks!
Happy Bassing!

yeah I already play bdouble bass in one of the orchestra I play piutside of school..I play bass guitar in my jazz band and for fun and my cello in my normal school orchestra