Kneeboards tend to be a fairly personal thing. Some people like complicated tri-fold ones with spaces for pens and torches and maps etc etc, while others find a cheap clipboard from the local newsagent works just fine. Things like leg length and amount of space in the aircraft you fly also have a lot to do with it.
In the hunt for the perfect kneeboard, I used to have four; I'm now down to two, one for f/w flying and a smaller one for rotary.
So I'd say you should buy the cheapest one you can find until you've done a bit more flying, then see what suits you.