The very outboard footstraps and wide style of the iSonics will feel odd at first but you will very quickly get used to it and the way they plane through lulls is superb. The iSonics also have a wider wind range compared to the s-types giving you more time on the water. Gybing is about the same on both not that I'm much good at it.. The S-type is more nimble in the turn but the iSonics have more width in the tail to keem them going. Tacking is a different matter. The iSonic has very little volume in the nose so you really have to get round the front quickly. The S-type has a bit more float up front but you still have to be quick. Upwind the iSonic is almost up there with formula kit when powered up. Last weekend I was out on my iS115 and managed to totally stuff a formula guy.
If you go for the iSonic then volume is going to be a hard choice. In 6.5 weather my iS87 is great and suprisingly easy to sail in light winds that I would not normally consider going out on such a small board. The 115 I use with sails from 6.5 up to 8.5 and when I've not had the smaller board with me I have even used it with a 5.5 and it was still ok.
I think Ian Fox would be the best man to ask when it comes to choosing the right volume for you.
If you are into a bit of jumping then the s-type might be a better option and in certain conditions can be faster.
I'm sure there must be somewhere near you that you can demo some kit. Give it a good go on both before you decide as they tend to grow on you. First time I tried the iSonic I wasn't too sure about it but by the end of my second session I was hooked.
Are Tushingam popular down your way? Here in the UK they are one of the most populer brands faring extremely well against the more expensive brands.
Out of interest, why are you trying to gain weight? Most of us want to loose some, Ive been trying for ages to get from my current 85kg down to 78