RE: Isonic 2007 -> 2006 changes
For 74kg and 5.8/6.5m the iS87 is the better choice for powered blasting and slalom use (speed obviously too, comparing these two)
It won't carry a 7.5m that well, and if you're running GPSS 1Hr and the wind is a bit patchy, then it maybe easier to maintain a higher AVERAGE (although lower peak) on iS94 -unless you're really sure to be powered, in which case you will have an easier ride (a big deal for serious 1Hr sessions) on the smaller is87.
iS94 is a serious weapon in the 6.5-7.5m range (and useful in 1Hr mode as above) but might be getting alittle big for 74kg in true 5.8m conditions, unless you have pretty flat water in that wind..
Hope that nails it, bit less us know if you need more.
Cheers ~ Ian