RE: Replacing my S-type 115
Well, if it's a choice between the ST93 and the ST104, I would think it best to consider your most often sailed conditions. If the likelihood of powered conditions (4.7 to 6.0) is more often the call, I think the ST93 would come out ahead. However, if conditions in the 6.0 to 7.0 range are more likely, especially if the chances of a sudden lightening of the wind are also part of your normal scene, than the ST104 makes more sense because of the extra volume. In reality, the two boards are actually very close in dimensions, hence the same recommended ceiling sail size (6.5).
I guess there's another thing to consider in the mix, and that would be the water conditions (swell and chop). In my locale, the wind and water conditions can sometimes be unrelated, as the swell can be coming from a totally different direction, or it can be coming from a great distance away. If you experience similar conditions, the smaller board would tend to be less flighty overall.