We'll be arriving in the morning, I imagine there's a morning rush-hour as well?
If so, would it thn be best to go to Bangkok City Air Terminal (Makkasan Station) and walk to MRT Petchaburi Station? [.
Recent BKK traffic has been HELL and morning is the worse than the evening- the train is a good alternative but you may want to try this option as it saves a lot of walking time
Instead of getting off at the terminal, take the train to the next stop which is the end of the line - from there, it's a simple cross over to the BTS which drops you off at the hotel... an entrance from the BTS to the lobby!!
it may take a few more minutes but it saves a LOT of up and down..