You will also be required to pay the 700 baht departure tax if you go landside. It is most likely that you would not have paid this tax when you bought your ticket since you are connecting only. Just something to be aware of.

For some bizarre reason my award ticket did not include this tax and I had to go through the trouble of paying it now as I was checking in. It's a lucky thing I had extra money left over.

I would opt to go into town too with an 18 hour layover. But you will likely be heading back to the airport at rush hour so I would add a bit of time to the trip.

At 9:15 tonight the taxi fare plus tolls was about 300 baht. If you are taking a taxi from the airport and use the official taxi stand the lady will ask you where you want to go. They write it down and give it to the driver. So I wouldn't worry too much. At 11 at night traffic should be light and the trip 30 minutes or less into town.