The futura is to my opinion more forgiving to sail, easier to get the performance out of it for the intermediate windsurfer. Coming from the shark 145 and 6.8 being the most used sailsize, I would go not too big. The futura 111 may be a good solution, but also the 122 should do the job. The futura 101 is also an option, but the difference between this board and your old shark could be large.
Asking the question about the reason why buying an isonic is also aswering it: your level of sailing (it's a guess of course) will grow buying a futura!
(I own the 101, love it