The AOT limousine service is a rip-off. I would take the train to save on the costs. I may not be quicker as the taxi is likely to get caught in traffic on Asoke/Dindaeng road from Petchaburi MRT to Sukhumvit in the early afternoon. Actually, some times I take the MRT from Petchaburi to Lumphini st. and a taxi from there (on Rama IV) to Conrad to avoid the likely traffic jam on Asoke/Din Daeng rd and the issue to flag down a taxi at Petchaburi st. It is never an issue to get a taxi by the entrance to the MRT on Rama IV.