Assuming you have a heat source capable of heating five gallons of water, go with the 24 quart pot. A nice boil has a way of boiling over quickly, especially if you have just added extract, and you do not want to have to clean up that mess. A bigger pot will let you do full boils with a little room to spare.
A few weeks ago, I was at a Wal-Mart with a grocery department, and I saw large stock pots near the Mexican food aisle for sale, presumably for steaming tamales. They were at least six gallons, and being sold for $25. It probably wouldn't be as nice as the other pots, but it might be worth a trip to Wal Mart.