I have both, and generally prefer the unlined basses. I learned how to play bass on an unlined fretless, so that is where I am at home. Part of it is the look of a clean unlined fretboard, part of it is that I like the challenge of getting my intonation right without the aid of lines. That's just what I like though.
I voted "carrots" because, as a die-hard member of the Carrot Party, I always vote "carrots". However...
Lined
vs. unlined. I have to go with "lined". For one, I've been playing fretted instruments for... well, since 1968. I never even had a fretless bass in my grubby paws, until last October. Reverb had a left handed, '93 Canadian Fury Anthem lined fretless bass that just screamed "If you don't buy me, you ain't got a hair on yer..." well, you know. It's an absolutely marvelous instrument, and once I got the hang of putting my fingers
on the lines instead of behind them (took me about 5 minutes), I've been playing the snot out of it ever since. I like lined fretless basses so much, I bought a G&L Kiloton Lined fretless about 10 days ago. Birdsong Guitars is making me a killer custom Fusion lined fretless, and I have a Schecter Stiletto Studio lined fretless on order from
Sweetwater. But, why lined instead of unlined? Well... in Junior High School, I was banished from Music class because the teacher declared me to be tone deaf. I don't think I'm
completely tone deaf, but... I'm probably pretty close. So, trying to play an unlined fretless - especially now that I'm in my '70s - would make for a very steep learning curve. And, one that isn't likely to be a whole lot of fun. At my age? I don't have much use for "not a lot of fun"...
So... bottom line? I can play a lined fretless bass
right now. But, an unlined fretless bass? Well, maybe - if it was 1968 - and I wasn't mostly tone deaf? Maybe; but how many left handed fretless basses were there, in 1968?
