Both, I think, can make for an interesting element in a story. The main difference is that the hail storm would "erupt" within a short space of time and would be a disaster more easy to be "recognized" as such.
A heat-wave is in a sense a more "subtle" disaster (in spite of the fact that its effects can be much worse than that of a hail-storm). When some years ago (I think it was in 2003) a heat wave struck here in Europe the River Rhine became unnavigable because there was so low a tide. Wood-fires broke out (Portugal, Greece, and France being hit by it in particular) and more than 50,000 Europeans died from the immediate effects of the heat wave. Nevertheless it was not seen as as much of a disaster as for example an earthquake with a lesser number of victims would have been. Many (not all) of the victims of heat-stroke were older people who could stand the high temperatures less than the younger people could.
With regard to an LBT story a heat wave is tricky as it begins (macabre as it sounds) with "fair weather". If elements such as lacking water and wood-fires are included there is a strong risk of it being very repetitive of LBT 3 where all this was already there. It could be interesting (but would also have to be treated with great care) to have a character suffer a heat-stroke.
Imagine Littlefoot's grandfather suffering a heatstroke possibly while standing in the broad sunlight. Nobody could possibly move the huge longneck into the shadow to help him. One idea which I really like would be the reaction of Cera's father in such a situation. In spite of everything (his passivity in LBT 4 most notably) I think he kind of admires Littlefoot's grandfather (not to mention the fact that he owes him) and I could imagine that faced with the situation of Littlefoot's grandfather's life at peril and him being able to help he might go beyond himself by trying to get the shadow (and water) to grandpa if grandpa cannot be brought into the shadow or to the water.
As for the hail the effects would be similar to that of some other disasters we have seen in LBT already (e.g. storms with flying debris). One thing I'm wondering about is whether or not a heat-wave would actually exclude the possibility of a hailstorm. Hailstorms can occur in summer (though I don't know for sure if they could occur during an actual heat-wave). It could be a somewhat "bitter" scene if during a heat-wave the dinosaurs spot clouds in the sky and while joyfully expecting some blessed rain they are suddenly showered with particles of solid ice.
PS: I'm going to change the thread's title from "The Land Before Time XV" to "Heatwave or Hailstorm?" to make the title less misleading and more to the point of this thread.