What, functionally, is the difference between the effect on Rooftop Storm and cards like Omniscience?
The ruling on Omniscience states:


  If a spell has Variable Colorless in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.


This implies that the alternative casting method allowed by Omniscience is identical to casting with a cost of 0, which is what Rooftop Storm does. Trinisphere still functions on cards cast with Omniscience.

It seems like the difference in wording is mere flavor, and not the difference between cost=0 and cost=null.