It is always a good idea to test ride or try if possible if you've any doubt or uncertainty. But agree that is often not possible or practical, and that is one of the main drivers for offering detailed and accurate customer (potential customer) support and recommendations etc.
The on water performance size [note, not only volume based
] of these boards is actually quite close to one another, it's not a quantum gap, which actually makes the differene in selecting one over the other maybe less critical (at least in that perspective).
Considering all of the factors that seem relevent to this discussion, if 6.7m will be the focus size for the new board, then the ST104 will be choice ; if you plan to sweetspot the new board on 5.7 conditions (ie : so moreso than 6.7) - and you are comfortable using the "big sail/small board" combo then the ST93.
Obviously, there are benefits and compromise at both ends of the range/s; maybe another way to describe this is to suggest the "Sweetspots" of the boards (in your case) will be ST104 at 6.7m, ST93 at 5.7m.
One of the factors influencing the level of compromise (especially at top end) is water condition for a given wind speed/sail size; as you described conditions of choppy to flat, the "chop " factor obviously varies accordingly.
Hope this sorts it, but let us know
Cheers ~ Ian