If your main goal is to go as fast as possible with your light weight, I would strongly recommend to get the speed board. I'm 65 kg and have been using the iSonic Speed W53 with mainly 6.6 and 7.7 m2 sail, wind range 12-25 knots of wind. This is a great board from flat water up to medium chop with 50 cm swell. The biggest speed difference with regards to a pure slalom is when going downwind at 120-150° : the true speed board has a really big advantage for a light weight rider with her overall smaller "footprint", smaller tail block, smaller fin, reduced lenght and swing weight, reduced volume and lighter weight. There is no limits, except that of the pilot himself. If I would buy again a speed board, I would pick up the W53 (or W54) without question : she's a true gem with 7.7 m2 sail and ±30 cm fin.
If your main wind range is 50% with 5.5 m2 sail and 50% with 7.0 m2 sail, I would pick up the iSonic speed W49 board because of your light 62 kg weight. You obviously want to get the smallest gear going in full control at full speed with a 5.5 m2 sail in ±30 knots winds. In those high wind conditions, excessive volume is just an useless balast. However, if your main wind range is 20% with 5.5 m2 sail and 80% with 7.0 m2 sail (or more), better get the W54 for medium to light wind speed sailing.
Finally, if your main goal is to win slalom runs in any water condition and keep top notch control during jibes, I would choose a dedicated slalom board à la iSonic 80. Higher scoop-rocker line, wider tail block and softer rails all do enhance her jibe control and speed exit quite significantly. The more difficult and severe chop it gets, the better the iSonic 80 goes.
I'll ask Christine from the french forum to churn in and let her to tell us what she's experienced with her light weight 62 kg.