Hi Paul
I'm not sure if I have the narrow bucket SES. The inside measurement of the top of the bucket is 39cm in the SES and 37 cm in the Spark. More importantly though, the Spark has a cup shaped bucket, so it narrows in a curving fashion towards the seat, whereas the SES is much more square in cross section. The Spark also has the back section sloping away from you. This has the effect (together with the hump) of centering your bottom very securely in the middle of the seat. And eliminates coccyx rub. The for'ard part of the cockpit is also very narrow, creating an overall very snug fit. I think this is the most important component of the immediate sense of security, control and hence stability I get in the Spark.
Both boats are relatively low displacement, but the Spark much more so. That long, almost needle shaped nose has very little windage, and slices through chop rather than going over it.
Stability is difficult to separate from fit. I suspect the SES has better primary stability and is similar or better in secondary. I base this on the increased number of "fresh air shots" I have to take when paddling side chop in the Spark. However, despite the lively rolling, I never felt I was going over, possibly due to better control. Someone who fits their SES better than me could react to changes in attitude more quickly and positively and effectively experience better secondary stability than in the Spark. I can't be sure; in summary for me, the SES is better at primary, the Spark better at secondary.
All Stellars turn sluggishly for me. The Spark feels like it could turn on its length. With the stock rudder!
Both boats have low sides. When the water comes in, the Spark drains much better than the SES, even with its bullet. And the less said about that stupid bung, the better. The low sides make remounts easy, the SES possibly easier, due to the better primary.
The SES has a beautiful, tough level of finish, the Spark only so so. The SES has far fewer visible scratches than my other, less used, boats.
Both boats are technically elite boats for small people. They feel very different, so, as Greg says, you definitely need to try them out. On the other hand, it won't take extensive sea trials to make a decision! I was smiling, laughing, within my first 100m in the demo. I doubt I'm going to be much faster in it and I don't think it is intrinsically more stable, but the Spark makes me want to rip turns, slither sideways on waves like breaking out the back wheel on the motorbike, leap onto every wave in sight and generally hoon about.
Past skis: Spirit PRS, EpicV10Sport Performance, Epic V10 Elite, Stellar SES Advantage. Current skis: Fenn Elite Spark, Fenn Swordfish vacuum. Custom Horizon, Epic V7