I stayed in the main building and have no experience with the other building. No elevator which was fine with me, but something to consider if stairs are a problem for you.
I have always stayed in the old section of Bangkok by the river, but decided to try Wendy House based on a friend's recommendation. I really liked it here. The rooms are basic, but very comfortable with each having a refrigerator and a/c as well as a hair dryer and kettle for hot water. They have laundry facilities that are reasonable and do a great job. Wifi is great and breakfasts are really good with a large selection of fruits, pastries, cereal, rice or noodles, as well as eggs made to order. Wendy will help you with tours and information on getting around and the location, while on a quiet soi (alley) is very close to MBK and Siam Center and the BTS Skytrain. From the BTS, you can get pretty much anywhere in the city by connecting with the MRT (subway), the river taxis, the train station, etc. I definitely plan to stay here on my next trip to Bangkok.