Go the 100L. I ride a 99L Mistral Joker in everything from 12kts to 40kts (flat water) and it's no issue; I weigh 72kg. The extra touch of float will also help you when you start on the new tricks, slightly more forgiving if you don't slide all the way through a move etc. Depending on the brand some will handle chop better than others, the Joker is actually pretty smooth for a 99L board but some others (the flares?) can be pretty harsh in chop. That said when you are learning anything that involves sliding you want the flattest water you can find, imo.
Another thing to consider is that with the 100L you can run a sail size down earlier than you can on a 90L. I'm often cranking around on a 4.7 when others are on 7m slalom gear, so the crazy low planing threshold of the 100L is a plus.
I would say the advantage of a 90L comes into play in the aerial tricks as they are a bit easier in the air but when it comes to bawse spec sliding, you can't beat a ~100L