We did an open bar by default because that was all they offered, but I think we made out like bandits considering how much our guests drank. I can't count the number of times tequila shots went around and if we were paying per person it would have made me nervous. Factor in the mixed drinks, beer, wine, & champagne and it would have got really pricey.
In your case I think an open bar makes sense too. What about cocktails only for the cocktail hour, open bar for the next two, then beer/wine only for the last? That may help with the costs a little.
If your budget allows I definitely say cocktails for cocktail hour, even if that means limiting it to beer/wine for the rest of the night. Good luck!