Answer: Hybrid willows is a general term that could include many different trees and shrubs with the botanical name Salix. Generally speaking, willows prefer a wet or moist soil to perform at their best but will tolerate average soil moisture. Also generally speaking, willows are weak wooded and will break apart in severe storms. They are also known for sending out roots over a wide area to seek out water. If you have a wet area, willow might be a good choice.
Rose of Sharon or Hibiscus syriacus is a sturdy, usually trouble free flowering shrub which can grow to the size of a small tree. It blooms during the summer. It loses its leaves each winter and is very late to leaf out in the spring which can be a liability for hedge use. It requires full sun and a well drained soil. It absolutely will not tolerate a damp, wet or swampy spot. On the downside, Japanese beetles may attack it. It may suffer some winter dieback in extra cold years in your area, although this will not prevent it from blooming you may have to trim it a bit each spring to remove any dead branches. (Your zip code places you in winter hardiness zone 6A, the coldest part of zone 6. If the site is exposed and windy, it may actually be as cold as zone 5.)
These plants do not look the same at all. I would suggest you try to see some growing in the landscape to see which one you like best visually. You would also need to know a little more detail about the willow in terms of expected mature size and overall shape.
Your local county extension and professional nursery staff should be able to help you determine if your planting site is better suited to one or the other of these. In the meantime, I hope this helps you in the decision process.
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