Commerce doesn't do very much to improve Venice's GPT. All your international trade routes should be cargo ship in the long run, so Exploration is better for GPT.
Rather, Commerce is better for incidental gold events - doubling Great Merchant trade mission yields (which is quadruple for Merchant of Venice) and saving gold on rush-buys. Both of which you have to maximize Merchant generation - build bank ASAP, build merchant point wonders, work all your merchant slots in the capital, get Great Person multipliers, etc - to get the most out of. You need to go on trade missions as well as buy up puppets (which is where you'll be doing your rush-buying) to get the most out of commerce.
If you aren't generating a lot of MOV's, all Venice gets out of commerce is the trading post bonus. In which case Exploration is better.