The needs of the particular species. Some tarantulas are more delicate than others and require a bit more care. For example, some adult species you can leave for three weeks or more without a worry, but others need checked on every few days, mostly due to an increased need for humidity. This is a concern for those who travel or vacation and do not have someone who can watch their spiders. Generally, species that require a warmer, more humid environment are higher maintenance whereas species that thrive in dry conditions are usually more hardy and require less upkeep. Though some adult species do great for a time without any maintenance, that does not hold true for most spiderlings. If you have a spiderling, it will require misting and feeding on a more regular basis.
Size. Some species require larger habitats than others . . . consider how much space you have to place your collection.