A druidic animal companion horse
Your cavalier gets neither the light horse or the heavy horse as a mount; the cavalier's special feature states that:
This mount functions as a druid’s animal companion, using the cavalier’s level as his effective druid level.
Thus, we refer to the rules for the druid's animal companion, which has its own list of creature statistics to use as the base stats for such animal companions, which are then improved by adding the appropriate bonuses due to the druid's (or cavalier's) level.
The horse animal companion has the starting statistics:
Size Large; Speed 50 ft.; AC +4 natural armor; Attack bite (1d4), 2 hooves* (1d6); Ability Scores Str 16, Dex 13, Con 15, Int 2, Wis 12, Cha 6; Special Qualities low-light vision, scent.
These horse stats don't agree exactly with either the light or heavy standard horses, but of course your mount will rapidly outclass such ordinary horses as you gain levels and it grows in hit dice and abilities. You might be especially interested in using the charger animal companion archetype, too, since that synergises with your cavalier abilities.