opinions... cash bar or fixed price?

Hi,

I am getting married not in a hall but more of a restaurant and I have the choice of either doing a cash bar of $6 per drink(my FH and I would pay for it of course) or a open bar at $26 per head. We have 220 guests and I would say there is about a 30-40 person drinking crowd and the rest are typical wedding guest drinkers then old people and kids. If I do the open bar at $26 per head, 20% will automatically be added to the total. If we do the cash at $6 per drink, we can give what we want for a tip. Has anyone had experience with a cash bar before at their wedding? Our wedding is on a Saturday night in June on the beach and many people are staying at the hotel where we are getting married including both our families and our entire wedding party. We are trying to save a few dollars(aren't we all) and thought maybe the cash bar would work in our favor but am not positive. We had a Saturday Day engagement party in May last year and we did cash bar for 125 people at $5 per drink and we only ended up with a $1500 bar tab. 2/3rds of the guests came from out of town and had to drive home. Thank!