Kindle Fire HD advantages: Slightly nicer (to my eyes) screen, HDMI output, far nicer built in speakers.
Nexus advantages: Better performance overall, far more open, far easier access to far more apps, proper Android, Google Play store.
Since I have a smart TV with nice, built in goodies like Netflix, the Fire HD's advantages are moot to me so I would go for the Nexus (which I already own, in the original 16GB incarnation, BTW).
If going for the Nexus, unless you intend to use it exclusively in the home, I'd regard not going for the HSPA+ model as a false economy (though I note the HSPA+ model is about as rare as rocking horse ---- at the moment).