Sun corals are definitely the way to start if you plan to continue with more/harder nonphotosynthetic corals. They are more difficult than dendros, but in that sense they serve as a good bellwether of the feeding and maintenance that will be required for harder corals. With frequent feeding, they thrive, grow, and multiply pretty quickly. Thus, you can easily tell whether you're doing a good job. Plus, they are reasonably cheap, as compared to dendros and especially rhizos.