Nortriptyline is the active metabolite of amitriptyline.
Therefore they are the same therapeutically.

Amitriptyline came first, then when it was discovered, Nortriptyline came later.

http://en.wikipedia.org/wiki/Nortriptyline

We have some other drugs today, which are active metabolites of the parent drug.
Allegra is the metabolite of Seldane (which was taken off the market for long QT risk ..heart arrhythmias.)
Clarinex is the metabolite of Claritin (antihistamines).

Elavil (amitriptyline) remains less expensive than nortriptyline and is often offered first. It has been on the market much longer than nortriptyline. So doctors remember it first. Pamelor came along just before SSRIs and therefore was not a big seller for depression. The timing was bad for it. But it was promoted as an adjunct treatment for ADHD children. It gained some bad press for long QT syndrome, and some children given too high of doses died of cardiac arrest. The treatment of ADHD never included Elavil... however. So I guess doctors just think Elavil is better, but it is not really....it is just older. Elavil was used as an antidepressant for a very long time. The chemistry of drug metabolites was not elucidated until more recent years because the technology wasn't yet in place. Today some papers claim that nortriptyline has fewer side effects, and that is why it gets prescribed for pain which is still "off label" BTW.

Both Elavil and Pamelor affect QT of the heart. But in the doses used for neuropathic chronic pain, this is unlikely, unless one is taking other drugs with it to potentiate this effect or if one has genetic long QT syndrome. Very low electrolytes --low potassium and low magnesium can also be additive for long QT.

One other thing, Elavil has potential to alter blood sugars, mostly by raising them. I had to stop a trial of this last year in fact. So diabetics and pre-diabetics should test themselves to see if they get this effect.
It does not happen for everyone, just some. In fact for some it may lower blood sugar readings. Strange but true, and wasn't understood until recently:
http://www.drugs.com/amitriptyline.html

Big Pharma has been using this type of technology more often.
In the case of Nexium vs Prilosec...it reached levels of fraud.
This Wiki article explains:
http://en.wikipedia.org/wiki/Esomeprazole