When choosing a stand up paddle board for a SUP race there are two things to consider: Stability and Speed.  The narrower the board, the faster the board.  The longer the board, the faster the board.  Everyone wants the fastest paddle board out there, but if you continually fall off because the board is too narrow, it will do you no good.  Make sure that the width is wide enough for your skill level.  If you are just starting out, chances are that you wouldn't choose a 23" wide paddle board.  Hull design is also a factor in that some are specifically designed for downwind races, some are designed for chop, some are designed for stand up paddling in smoother water.  Talk with other experienced paddlers or your local paddleboard shop when choosing the best paddleboard for you.