Listen to last night's class, on the nusah of Birkat Nahem, here.
Follow along with the sources here.
For further research:
1) Read Yael Levine Katz's thorough article on this topic in Tehumin here.
2) Read R. JJ Schacter's analysis in YU's Torah to Go here.
3) Read R. Mordekhai Sevi HaLevi Sion's analysis in his new book BeShuvi LiYerushalayim here.