Answer: In general these are both easy care plants. Sweet William are bushy plants, 6"-18" tall, with cushiony tufts of lance-shaped leaves and tightly packed bouquest of blooms. Space about 10" apart and provide rich, moist, well drained soil (meaning not soggy), in site receiving full sun. Dianthus can be divided about every third year to rejuvenate the plants, or you can simply start new replacement plants from cuttings when the originals begin to look shabby.
Perennial carnations grow 6"-24" tall and should also be spaced about 10" apart and given the same environmental conditions. Cut back spent flowering stems for additional flowering throughout the season.
If you are looking for more detailed information about perennial gardening you might take a look at some basic books. One I particularly like is "Perennials for Dummies" by Marcia Tatroe, ISBN 0-7645-5030-6.
Q&A Library Searching Tips