I Have sailed the 115 S type & the Futura 111. I am also 80-85 kg. The problem with having only 1 board is compromise, but offcourse you already know this. In my opinion the 111 would suit 7.8/6.6 & the 101 6.6/5.4.
You can however extend the range by getting another couple of fins. For example if you went the 101 way, you could use a nice 40cm fin to help get and keep the board planing in marginal conditions. I would also point out that if you kept your weight more towards the 80kg, I would highly reccomend going for the 101.
As far as how the boards compare in terms of sailing, the Futura is more forgiving than the S type ,planes earlier, and in my opinion handles chop way better and turns like the old carve 111 did. So from that I guess you can believe the hype from starboard this time.
In summary if it was me I would go for the F101 because:
- 20-25 knots is the best fun
- 10-15 knot days can be frustratiing regardless of board size
- then F101 is going to be way morer fun than the Stype 115
- challenge yourself with early planing techniques and fin selection
- eat a few less big macs...