With Avraham we see a lot more need for reassurance and arguments with G-d. It is almost like on the fundamental level Avraham does trust G-d, but every now and then, he still has moments of doubt. When things don't work out like he thought they would he needs Hashem to physically remind him He is there. For example when Lot left, or just the fact that Hashem talks to Avraham A LOT.
Yitzchok seems to understand that G-d is completely in control, and never doubts that. He doesn't need the constant reassurance, and is willing to ask G-d for a child. He "understands G-d" at least as much as is possible. He follows what G-d wants him to do without argument, just complete faith.
They both had different backgrounds: Avraham lived among people without his ideals, whereas Yitzchok lived with Avraham. I think that it is much easier to start something new, rather then to follow the path of your parents. Most people want nothing to do with the way their parents raised them, yet Yitzchok does what his father did to prove that he would do it just as well, if not better.