Wow, your timing on this is impeccable because I was just logging on to recount my experience from first week playing Ibanez SR370EF fretless. I've played fretted on and off for 52 years, more off than on. Total "on" play time about 18 - 20 years. Intermediate level, hobby level bassist, learned by ear initially, now versed in intermediate level theory. Solid fretted player, play jazz finger style and good technique and physical skills overall. I found the Ibanez very easy to play, after a couple days of it feeling alien to me, board looked weird, even though it is basically the same neck layout as my Ibanez SR1350B Premium fretted. I would say lines are helpful for orientation but will become secondary, or nearly worthless, as you get much more used to the instrument. It's really more about muscle memory and more important, ears. After 5 days, only 5 days, I can play nearly as well, though a little more slowly and I have to check position more often visually. I'm off a bit above the 15th fret but probably becuase I don't go up there often enough. I think with more practice I will be able to play without looking, as I can with my fretted basses.
There are those here on TB who say you have to be able to play without looking before trying fretless, I say no. Also those who say to play with a tuner attached to check your notes constantly. Again I say, no. Ears are better. Check out Tony Franklin's video on YouTube about whether lined is better than unlined. Scales are not linear, you have to make adjustments, tuners are useless after getting your open strings in tune and making sure your intonation at 12 fret is on the line. Tony demonstrate his being "in tune" on thirds gives you a flat sounding note. That's why we have weird angled fretted instruments?
As I write this I wonder if a better fretless than the SR370EF would be significantly different but I can tell you this is one fine instrument and a value at the price point. I'll bet the Sire is fine too. Just be sure to put flats on ASAP. My SR370EF came with rounds, and board already had some damage at 24th fret line. Or tapes would be fine too.
I may at some point upgrade from the SR370EF to a better fretless but it will keep me happy and challenged for awhile to come. $475.
Oh, pics are uploading sideways again!!