And unless you really don't like it, I'd recommend buying what the school is recommending, since it's likely to suit the style they'll teach you to use it anyhow.
They aren't expensive, and if your preferences change you can get a better / different one later.
For the record, I use a Pooleys helicopter kneeboard and love it for most of my fixed wing flying. At your current stage, don't bother with a GPS mount kneeboard, you won't be allowed to use it (the GPS) anyhow.
G