Both are good for biking, in fact in Aventura there is also a store about bicycles and equipment.
Oleta River Park is located at 163rd street, close to Collins Avenue, Sunny Isles there are trails for biking and there are many people riding on weekends there.
Also there are groups that ride bikes in Aventura for route biking but they go from Aventura to Sunny Isles and back.
It depends on what you want, also Hollywood path on the beach, but there is recreational biking there.
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