If you're not in a hurry of some sort, take your time and try a few boards. Both F and iS will feel very different to your Bic. Especially with iSonic, you'll need more than a few runs to get used to it. It's not an easy answer, because I have some friends who really like "sharper/harder" ride of the slaloms (compared to freerides), while others hate it. Volume-wise, at your weight iS111 might be just enough (it's wider than a Bic, and it will take an 8.5), but it's worse when schlogging (wide and short). So it depends on how much lulls do you have on the lake, among other things.
Maybe you already know, but in the end it boils down to this:
Freeride = a bit easier to plane, easier to schlog, easier to gybe, more user friendly.
Slalom = you need to work a bit to plane (after some time you get used to it), higher top and average speed, better up/down wind angles, edgier/scarier/more exciting/more demanding ride, a bit more technical to gybe (but fast and rewarding!). For 7 or bigger sails, on a lake, I prefer slalom over freeride by a wide margin.
There is some learning curve for slalom, but all this is not out of reach for casual sailors, it's just what feel do you want from your sailing.
PS Btw, 3-cams freerace sails will do just fine on both F and iS.