Baan Klang Nam is very, very different from MahaNaga. While the setting of BKN by the river is very nice, like most Thai restaurants it lacks in decor and overall ambiance. If you go, reserve a table by the river and NOT in the house.
BKN is also more traditional Thai (not fusion like MahaNaga) and it is significantly cheaper (2 people can easily eat here for ~1k baht).
Totally different places. Try them both! (but I'd choose MahaNaga for St. V's)