Depends which way you choose to travel. I have tried the train and it took me 1hr door to door, at that time of day I would allow the same for the taxi but it realistically will take you around 40 mins if there are no dramas. You may find that time useful so you have time to chill on your preferred departure lounge should you have access.

For me the 2 changes of station on the sky train are not worth the hassle when the travel time is the same or longer. If you are price sensitive the train is definitely the better option - around 100 baht ish for the entire journey (you need to buy a second ticket at Phaya Thai for the airport line. The taxi will run you roughly 250-300 + 70baht for the highway.

If you do take a taxi, insist on paying by meter - the door staff at LeMeridien should do this for you.