I think that it all comes down to what you want to do with the board.
When you said you were going to use sails from 6.5 down to 4.5 and also taking your weight into account, I assumed you were talking about 25-40 knot conditions. These conditions normally deliver a lot of steep chop unless you are sailing in a really well protected bay or with offshore wind.
I heard that fhe futura handles the chop quite well, but I'm convinced that a FSW or wave board does it much better.
In high winds on my home spot I sometimes start off with my IS87 and a very small 5.0 Race sail (North Warp).
If there is too much chop and I can't handle the IS87 anymore, I often only change my IS87 for a pure wave board (Naish 85) but with the same 5.0 sail and guess what. It works ! The much thinner and narrower tail of the wave board allows me to sail comfortable again with the same 5.0 race sail. Of course, I prefer to be also on a wave sail, but yeah sometimes I'm too lazy to rerig
On the other hand, when you are sailing in well protected waters or in chop that still invites for more speed in stead of less speed, the smaller futura is probably the way to go.