I've just bought 18m Soul to replace 17m Core Free. It has more power & flies far more happily in less wind. Prior to the Free, I had a Zephyr which needed much more wind than Free for 3 reasons, it's got 5 struts compared with 3 and it's not a true 17m, more like 15.5. I compared it both with my 14m Enduro and then 17m Free when it arrived, finally the Zephyr is higher aspect so harder to trim in light wind.
The Core Free weighs 5.4kg and the 18m Soul 3kg. The Soul is new & I've never kited in such low wind, one session on foil, one on surfboard. It does have masses of power & loops well for kite of its size. I'm flying it on my Core bar and 24m lines. FS recommend 60cm bar and mine is 54cm so ideally I'd need something longer, but seems fine for now. Probably not so if using much shorter lines.