I have used the 100L Supersport a bit lately. We have the 08 Pro edition over here. It is a fast board, I mainly use it with a 6.5 V8 and it is a sweet combo. It planes fairly early, I weigh 72kg and find it good in around 15-20K, can become a bit of a handful once it gets stronger for my weight. Handles chop well, i wouldn't class it as the easiest jibe around but needs a firm hand and will reward being driven hard. I would say it is a slightly detuned slalom board which loves powerful foils and a rider who likes going fast. It ships with a 33cm fin and I have used a 37 in it and it still went very well.
I would reckon it would be fine with a smaller sail but I haven't used it with anything smaller as I have a 91L Supersport to change down to which is extra special - very nice board.
Haven't tried a Futura....hope this is some help.

www.seabreeze.com.au/forums/topic.asp?TOPIC_ID=56888

I've got the Futura 111, which is a great board, and pretty quick. I posted a review of the 111 on here also (see below).

I haven't ridden a Supersport, but I know those who have and speak highly of them. Only drawback (and it's a big one IMO) is that the Supersport has a powerbox and all the best fins comes in tuttle.

I also have the Futura111. Have had it for exactly a year now and can't fault it for the conditions I sail. Very fast, gybes nicely, quick to plane, etc. I'm looking at down sizing to the Futura101 in the very near future.