While I think Nightblood will be different from/stronger than the average Shard Blade, I don't think he's going to be THAT much more powerful. If he was why would Nalan give it up and not just use it himself? Unless his Honor Blade prevents that somehow.
We know Nightblood consumes investiture, but wouldn't that just be Stormlight, not necessarily the Spren? If Szeth is going to be a Skybreaker he's going to need to bond a Spren to gain their abilities. Nightblood consuming that Spren would seem counter productive.