In the SC titles that I have played, Siegfried has always been stronger than Nightmare. And before the butthurt NM players go on about better step, throws and oki in SCIV, the simple fact is SCIV Siegfried placed more often and generally higher worldwide than NM in its lifespan.
And don't think this is some strange cultist belief because I'm a Siegfried player. People who have played me offline know perfectly well that today my NM is very comparable to Sieg in terms of strength, I love both characters but Sieg had the better moveset already. =/
In SCIV, NM was better than Siegfried at throws, step, damage and leaps and bounds ahead in okizeme.
Siegfried had more mixups, better SG game, better mids, was safer and had 3B (=P).
What we have seen so far reflects this between the two characters and frankly is to be expected.
Now as Idle has so kindly said already, this is a new game and we know very little about how the system or the characters will prove to turn out over the next few years. Siegfried may seem stronger now, but that could be very far from the truth once we know more, in fact in early SCIV the french believed Siegfried to be a top tier, not entirely true I think you will agree XD. So far all I have personally seen as main differances between the two characters is almost exactly what I saw in SCIV, certainly nothing I would make a concrete opinion about, especially before release. Give it time man, NM looks sick if you ask me and may turn out to be a fucking beast! =)