I have stayed at both and many others in Bangkok. I much prefer the Peninsula to the Grand Hyatt although if doing business in central Bangkok the GH is better located. My number one choice would be the Mandarin Oriental for rooms, physical plant, service, and is on the correct side of the river in terms of access to sites, restaurants, and business. However GH is more centrally located. If I were staying in that area I would stay at the Four Seasons and not the Grand Hyatt, but again I would first select the MO - just love staying by the river and love this hotel.