The construction work is intended to turn a few more floors of the Krungthep wing into long-term apartments for expats, a very lucrative market in BKK at the moment (Conrad, Marriott, etc. are already on it). I believe they are promoted as being the only riverfront ones.

I've stayed in the Krungthep wing once some 5 years ago; at the time, any Krungthep wing room was considered superior to main building rooms, and were quite luxurious indeed (better than Conrad/Hyatt/SGS/WGS/ROS IMHO). All rooms are riverview and have a balcony, but the 'downstream' ones are fairly close to the bridge, and somewhat noisy with the window open.

The main wing was recently renovated. Coupled with the ongoing works in the Krungthep wing, this might make the main wing deluxe room a safer, if less exclusive choice. The Krungthep wing has its own entrance (almost adjacent to the BTS station, very convenient), altogether more serene than the main wing's lobby. Most of the hotel services are in the main building, however.