I went to Venice earlier this year and only walked by BV. If I intend on browsing or buying I always find it has extra charm to do it where the actual design house has its origins. E.g. Ferragamo in Florence, Fendi in Roma, so Venice is the place for BV to go!
In terms of price there is no difference within the Euro zone. At least BV has the same prices in Italy as in France or Spain. I know for sure that LV is slightly less expensice in France, but it is not much of a difference. The € lets you compare quite neatly.
However enjoy your trip! Venice is lovely, and after the Brexit the $ once again is developing in your favor