Your experience level. Some tarantulas are quicker, more unpredictable, and have more potent venom than others, making it wise to include them only if you have some previous experience.
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.