I prefer Saigon to Hanoi. Obviously both have their charms but I found folks in Saigon to be a bit more open. I also had numerous encounters with "fixed" taxi meters in Hanoi which can be rather annoying.
I also believe getting to Siem Reap is a little easier from Saigon as flights are more frequent than from Hanoi.
Nha Trang is an interesting side diversion if you want some beach time near Saigon. Although I've not stayed, friends have liked the Ana Mandara when they stayed there.