RE: Which Isonic (or S-Type) to replace my Carve 121?
The iSonic 111 is most likely to be the best choice for the sail and wind range you've described. The iSonics have a more efficent early planing than the ST's, which would be a good choice except you really have that condition very well covered with K96 - and no problem getting planing early. Also the ST's can't carry the same larger sail sizes as the iS.
The iS111 will be a very good size for 62 Kg in this usage range, not too small for your weight to be OK on the occassional 9m session, but also not too big to become a problem at speed in mid chop, nor too big to overlap too much with your FF138 (iS122 would overlap a lot in effective range with FF138).
You could certainly pickup a good price for a close out iS115 model in the 2006 series, and no problem there. These end of season models often represent really good value - and can boost the fun/$ factor too.
The new iS111 has a bunch of evolution features inc a wider and thinner tail cross section, which especially for lighter riders means a more reactive, more natural jibe, with less effort (or weight) required to initiate the jibe at speed (in addition to more efficent overall performance). There are other improvements, such as the wingers for clean water release and extra fin drive, new fin upgrades.
Hope this answers your questions, but please let us know if you would like more info.
Cheers ~ Ian