buying a board

[mcstreet]

hi, i´m 38 years old, i´ve never tried a windsurf board, but i bought an aquasport multiglide 4.1, few months ago. I don´t have no windsurf school near me, so i´ve been learning some basics with the multisport, as it as a lot of volume, i never have equilibrium problems, and learned how to gybe up and downwind easily, uphauling, and sailing upwind,normaly i sail in small lakes with winds 5-15 knots

right now, aquaglide is limiting me, so i want to buy a real board, i don´t know what board to buy, i was thinking in a GO155 2009 (550 eur) or an ISonic 125 2006 (400eur),

Do i have to pass through an easier step, like buying a big volume with dagger? such a bic core 293OD?

i weight 73-75 kg..

and usualy sail in small lakes with low wind speeds, 15-20 knots max..

i´m portuguese, sorry for bad english.

thank you