That value would drop if you don't think 250ml would be enough for a single drink for you though. If you felt the need to consume two cans per drink at 250ml each, vs being satisfied with one 320ml can, then the larger cans might end up being the better choice regardless of value.
7500ml total, with 203ml per currency unit on the second one.
The 250ml cans work out as the better value, and have the added benefit of likely being more drinks since you're probably going to go through a can at a time.