In my opinion the Stormfather offered his own perfectly reasonable explanation for the different way he behaves with Dalinar, assuming that he is indeed the same Stormfather. Gavilar ultimately let the Stormfather down because of the influence of the Vorin religion as well as his firm belief that he had been chosen by a god-like entity to ascend as a Herald because the Stormfather told him just that. The Stormfather acknowledges that Gavilar's Vorin upbringing was a primary contributor to his failure, and that in retrospect it was inevitable and should have been obvious. He also mentions that it was a mistake to be so open with Gavilar and to provide information so "easily".
It seems to me that the Stormfather rectified those mistakes with Dalinar. By being withdrawn and reticent, he forces Dalinar to fight tooth and nail for every scrap of information. Dalinar has to go on his own journey to uncover a lot of these higher mysteries rather than having the destination of ascendance presented early on. During this search for information, he's even forced to abandon the shackles of the Vorin religion and he ultimately comes to the conclusion that the solution to their predicament must lie within the recreation of the Oathpact.
I could go on but I'll leave it at that and see what you guys think. I am pretty firmly convinced that it was the real Stormfather, merely learning from his mistakes and taking a different approach with Dalinar in order to fulfill Honor's directive.