A healthy Italian hive will have a lot of brood year-round. The queen tends to lay regardless of weather or nectar availability. Most beekeepers that pollinate early crops prefer the Italian for this reason--more bees early in Spring when it is still cool. Because of the large number of bees it is important to FEED an Italian hive over the winter, or leave sufficient honey in the hive. They can survive cold temperatures, but need food for the plentiful brood.
A healthy Carniolanhive will have a smaller cluster of bees over the winter, as the queen will lay less when the weather cools. As a result, there will be less brood and the hive will need less food over the winter. This can be beneficial in a cold, harsh winter climate where it is difficult or expensive to feed larger quantities. Spring buildup will tend to be slower to start, but once the hive is built up, the Carniolan should be equally as productive as the Italian. The Carniolan may not be ideal for a hot, dry climate.