The 3Nagas has the better location - while the Sofitel is not far, you don't get the atmosphere of Luang Prabang as much. It's a little bubble of luxury that could be anywhere in SE Asia...
I would not recommend staying only at the Sofitel, at least split your time. There are also very good independent hotels in LPB that are much cheaper than the 3Nagas and very nice...

oh, and I did have the same problem with points not posting afterwards - took a few emails back and forth to resolve...