I would suggest looking into both options to confirm whether the prices are current and correct. Information in Thailand can often be inconsistant and requires some follow up. If indeed these prices are correct, I suggest going with the Novotel "Fresh Up" as the best value. It is a brand new 4-star hotel with very nice rooms and facilities. Novotel also has a day use rate of 4,500 baht between the hours of 8:00 AM and 6:00 PM. For more info, visit www.sawadee.com/hotel/bangkok/novotelsuvarna/
Your layover between 6:00 and 23:00 is plenty of time to see some of Bangkok. The best way to do this is to take a taxi to the nearest Skytrain station, which is Mo Chit. If you arrive on the weekend, you can check out the huge market below Mo Chit station. Be sure to go early though as it gets very hot and crowded by 11am.
From there take the Skytrain into Siam Square. This is the center of town with many malls and lots of Bangkok energy! Riding one stop further will get you to National Stadium station where you can walk to the Jim Thompson House at the end of a small road (Soi Kasem San 2). You will see a sign, or just ask someone. Jim Thompson House is a historical and beautiful teak house - you can take a tour or just enjoy the peaceful retreat at the cafe.
Also worth seeing and early in the day is the gorgeous Grand Palace. You can take the Skytrain to Saphan Taksin station, above the river. Walk down the stairs to the river and get a river-taxi upriver to Tha Chang pier. Walk to the front of the palace to the main entrance. Do not believe anyone who tells you the palace is closed unless you have arrived after 3pm. There are many smaller locked gates along its walls, but the main entrance is open every day. Also nearby is the impressive Wat Poh (Poh Temple), accessible by foot, taxi or river taxi at Tha Tien pier.
This should be enough to keep you busy and ready for a nap back at your hotel. Have fun! KK