RE: ST vs carve
It's possible to run more (than 7.2m) sail on the ST115, it's the usual situation of how a "sweet spot" (or practical) limit is defined, and technically that also invoves rider weight (heavier riders will find the use of bigger sails "less sweet" earlier up the size range than lighter riders [ on the same oversize sail ] ) - also the sail type and ride style/tuning all play a part. in my experience (92kg) the ST115 is still really sweet with 7.0m but strating to feel overdone by 7.5m (race/free race sails).
Both the ST and Carve lines have sail sweet spot ranges that "size" lower than equivilent volumed iSonics (for example), which is a result of the more narrower tails and softer rail shapes, which contribute to the easy ride and moves character of the ST/Carve.
> choppy conditions : Not much in it, slight edge to Carve based on comfort (see below)
> upwind : Again it's close, decider could almost come down to fin and or strap stance (demi slalom vs freemove/SuperX)
> speed : Edge to ST here !
> maneuverability : Edge to Carve here, especially in "natural" ease of jibe. ST can be great, but requires more input concentration (not itself always a bad thing).
> comfort : Carve. it has the more comfortable and "safest" FEEL but reality is once you got the trust in the ST, it's also a very safe board to blast on you can go a LOT harder than you think and still not get bitten, just the Carve feels safer initially. Softness of ride also minor edge to Carve.
Hope that helps, let us know if you need more info !
Cheers ~ Ian