The nature and severity of your hearing loss will play a large role in determining which hearing instruments are ultimately recommended to you. Your hearing professional can help you understand your unique loss characteristics, and explain the models that would best suit your needs.
Consider your life, work, free-time activities. What are the things you do that are most affected by hearing loss? What are the things, if any, that you're not able to do because of a hearing loss? Define your needs and set priorities. Your job may also be a factor. If you work outdoors in the elements or travel frequently, and are concerned about a hearing instrument's durability, you may want to consider a back-up instrument.
Sound quality is perhaps the most important consideration–it's why you're even considering purchasing hearing instruments, after all. Not every technological advance benefits every hearing loss, and it's safe to say that even basic hearing instruments can deliver appropriate sound quality. Consult your hearing professional–he or she will help you assess the level of sophistication you need based on a range of issues.