You really can not go wrong with either one. We alternate between the two depending on what we are looking for at the time, now we probably are 75% at Onshore. We owned at Long Wharf.
Some considerations:
Onshore units are slightly roomier.
Easy walk from Long Wharf to the main commercial street.
Easier car in/out at Long Wharf , if doing things outside Newport.
Great chance of water views at Onshore, difficult to obtain at Long Wharf.
Washer Dryer units in condo at Onshore, in Laundry room at Long Wharf.
Onshore units have balconies.
The choice often depends on what is important to you, we generally like the washer/dryer in unit for longer stays. 2-3 days it doesn't matter. Fall visits , balcony doesn't matter etc.