I've been lucky to have a go of the new Spirit Carve 950 with the 90cm mast mounted on a 4'10" Pace board recently. I was using the Speedster with the Freefoil board before and its the one I learnt on. I haven't done a back to back comparisons on the same day yet, but hopefully very soon.
By the way, I'm not experienced enough at foiling to give proper expert opinions plus I'm pretty heavy so, what I feel, might not translate to you, but anyway, here's my quick/short opinions so far:
It's faster overall on all points being stable at slow and high speeds, but definitely feels more stable at higher speeds.
Takes off is about the same so just as good in light winds but feels more balanced once going especially at higher speeds.
Less back foot pressure when hooking along, feels more balanced.
Upwind angles felt kind of the same with a more balanced feel in pitch control on the newer one but I think with the extra speed of the Carve, the speedster felt like it had just a might touch more upwind angle, not sure yet. I reckon a back to back test should tell me for sure.
Downwind with s curve turns are much better on the Carve, the Spirit Carve absolutely shines here, the speedster feels like it just wants to track straight but this is a good thing when learning, it did for me anyway.
Talking of tracking in a straight line, it felt a little more locked in on the Speedster but that also takes away from the carvey feel so the Carve feels better, and of course this means more playful. A more experienced rider would get much more out of it than I.
Essentially its a much classier ride so in my humble opinion, a better foil. It certainly looks classier.
I hope I can give a better analysis when I get more time on the Carve especially when I get more experience.
Cheers for now,