My advice for foil boards is go light but tough. You may be surprised at which 'premier' brands have had foil mount boxes rip out. You are looking for decent ding resistance, making a good balance between durability and lightness, and for me even more importantly, you are looking for a bullet-proof foil mount box. Construction should include extra care for mounting a high density foam box with the stringers routed through it. The boards I carry have solid, well-built foil mounts.
Your next concern is getting the right volume for your skill level, activity and conditions. For winging start with +20L over your weight in kilograms, for prone about half your weight in KG is a good starting area.
I would say I am also biased about the bottom contours, with the major concern being the ability of the board to bounce off the water, rather than edge and stick. I personally am not sold that double concave bottom makes a huge difference, compared to smooth rounded rails. The important thing to remember is that you are not riding the board - you are riding the foil.