Pretty much agree with J-M here.
On the lake, in < 10 knots of wind, a board with more "glide" and a centerboard will allow you to sail more often
and further....more time on the water.
Staying upwind in < 10 knots of wind on a regular shortboard (not the Ultrasonic 147 or an older formula board with a huge sail/rig/fin) is not easy.
The center board takes care of this issue and will work better with smaller (< 7.5 m2) sails.
Depending on your weight a true longboard (Phantom) or a good transition board (Rio M or S)
will give you the most time and fun on the water at your lake in very light sub planning winds.
Hope this helps,