The four stations might add ten minutes at most to your ride, but that is not significant. I have stayed at the Marriott properties several times each and the Soi 57 is my favorite property, over the JW and Ren. The views are spectacular, as a Plat you will likely get an UG to a two room suite with 1-1/2 baths, one of which has a huge soaking tub against a window overlooking the city. The CL is very good. The staff is wonderful. IMO it is the quietest of all the properties both in terms of not hearing noise in your room and there not being as much busy energy in the hotel and lobby.
One of the biggest problems with choosing a hotel in Bangkok is there are so many good Marriott/SPG options. Choose one and you will have a wonderful trip. Don't worry what might be elsewhere.
As for the palace area, when I am with Bangkok newbies a like to take the BTS to the tourist boat, take it the whole way up without getting off just to get an overview, get off at the last exit, meander over to Khaosan Rd, eat lunch at the restaurant on the main road with the pig sign above it, go back to the tourist boat and ride down to the palace.