Not really adding much that hasn't been said already, but I would agree that the 6m is a very different kite.
I bought the 10m at the back end of last summer as a light wind tool for the hydrofoil, but with higher wind twin tip boosting as a back up option. The kite blew me away. Super powerful (it gets me up on the hydrofoil in what feels like zero wind and I am the wrong side of 90kg) and incredibly stable in the air (I can end up with super slack lines at times and it just drifts back and powers up, whereas I had quite a few big swims in light winds last year on the LEI after losing line tension (my fault) and watching the kite plunge to the water) but also really controllable in higher winds on the twin tip. Mid 20kts is no problem at all at my weight and the 10m is fast enough for me. It is a jumping machine, with really good height and ridiculous hang time on tap.
I love the 10m, but found it quite a handful on the hydrofoil in anything more than 15kts or so as it has such a big power spike and I am still no expert on the hydrofoil. As I am enjoying the foil board so much, I got the 6m as a higher wind kite for the foil board. For that purpose, it is superb and I have had some really great sessions. It looks ridiculously small and is almost too fast, but still has a great bottom end and drifts like no other kite I have been on. I could not be happier with it for the foil board.
The second outing was actually in a decent blow, when I tried it out on the twin tip. I thought it would be a good time to spend some on it and make sure I clear the 5hr 'bedding in' period. It was really interesting as it was super fast and incredibly comfortable in the gusts, but it is definitely not a jumping kite, at least not at my weight for that wind. It does jump, but you don't get the same ridiculous launch and I felt much more nervous at any height as I was dropping much, much faster on the 6m. To be honest, that would be the case with any smaller kite (and it does look tiny) but it was surprising just how different they feel.
I could not be happier as the 6m is exactly what I want it to be - a higher wind foil board kite. I think it excels at that and I would be confident going out in a decent wind as it has such a massive range and does not have the power surge that you can get from the 10m. If you enjoy foil board and twin tip riding, the 10m is a really good solution for 7 or 8kts (foil board) up to high 20s (twin). If you wanted to spend more time on the foil board, as I do, then I would totally recommend the 6m as an excellent companion to the 10m.
I enjoy LEIs and Foils (although have only actually ridden the Soul) but am a total convert to the foil for my personal riding (both style and conditions). I absolutely love the Soul and think it is the ultimate freeride kite. The bridles look scary at first, but you soon realise that they are fine and I would personally choose the bridles over having to carry a pump around and worry about bladder punctures.
I am thinking about trying shorter lines on the foil board. I don't feel any real need to speed the kite up but I like the idea of a more direct connection feeling and can definitely see the benefits of shorter lines on a busy beach. What is the consensus of opinion for line lengths on the 10m and 6m Souls for hydrofoiling?