90v100: Hard for me to say since I'm light. But I've used both of them. The 100 is mostly thicker and I think the rocker is slightly altered. In practice they sail kind of similar for me. Ie, the 100 is not really more difficult (but on the other hand not earlier planing or anything either). You loose some feel on the 100 since it is thicker but it still carves well if speed is not to high. The thing with all EVOs is that they turn a lot on their rockers, so as long as speed is not to great, you can whip around even the super big ones very well. For example, the fisrt time I used the 100 I was sailing 5.3 cross on, 1m waves and Kombat 79 which worked great. But I backwinded now and then and didn't hit the lips so well. I changed to the 100 which felt huge on the way out. Like a ship. But on the first wave I hit twice as many lips totally effortless than I did on the K79. So again, regardless of size you still have all that rocker in there and it does its job until the speed gets high enough so that you can not even get the rocker in hte water properly.
For a bigger rider the 100 will indeed offer more. It is specially developed to carry both a big rider and a big sail. And according to Antoine it does.
The question is what it offers a mid weight like yourself? It could be that for you the 90 is rather all round while the 100 tops it over into the "slightly cumbersome"/speed sensitive category. I think the limit for when this happens will be highly individual. But I don't think Scott sails a 100 (in fact he is mostly on the 80 in light wind. But he's a pro and can schlog anything).
Personally, I'm 70 and have found the 80 to be the best light wind size. The extra comfort the 90 offers is not worth it for me. If I would regularly sail waves with a 7.0 I might get the 90, but I'm not. Up to 5.9/6.3 the 80 is floaty enough and handles both slow and faster waves, so it is just a so much better compromise.
My guess it that by the same token, the 90 will be the best board for you.