Hi Dimitris, you are really light, so maybe the Grip may be the better board, as we don't have a smaller Mamba than 78.....and the upper end range of the Mamba is not as good as the Grip. Question is, did you ever use a smaller board than the FreeWave 85/which sail sizes do you want to use on the Grip/Mamba. Because switching from a FW85 for the first time to a smaller board, you may be ok with the Mamba.
But I still think Grip is the better choice, especially for your lighter weight and 30+knots.