No personal experience, but I've only heard bad things about the transit hotel, Louis' Tavern Dayrooms (but what do you expect with a name like that?). If you're going to stay at the airport go with Amari.

Do note that at the times you mention traffic to/from the airport should be reasonably painless (~30 min). Taxis should be in the vicinity of 200 baht -- use the public taxi counter and make them use the meter. (Note: there may be queue leaving at 11 PM, as this is a popular arrival time. Arrange a ~500 baht ride at the limo desks inside if you want to avoid this.) Unless you require a visa, the only problem with going through immigration is the 500 baht departure tax.