I have owned Speed 2 thru 5 and then Sonic FR, Sonic 2. Happy to chat -pm me.
The newer Flysurfers are chalk and cheese compared to the old models. The Speed 5 was a big step change to the earlier Speeds with a much firmer bar feel (closer to LEI than previous models). For twin tipping the Sonics were a nice upgrade again to the Speed 5. Probably not that great for downwinders on a surfboard but anything is possible. If I was buying new I would go the Soul over a Sonic for all round versatility, but a secondhand Sonic still offers massive performance for relatively affordable prices.
Have to agree with James. I have not tried the Soul yet, but have compared all the Speeds up to 5, and I have to say the Speed 5 is a quantum leap in terms of bar feel, responsiveness and agility. Looping is way easier and far more responsive than the Speed 3, and even the 4. In fact, I think the Lotus 4 was really a Speed 3 (3.5?). But the 5, is a huge leap.