If you would base on the map, Fiji and Bali are located in two different locations. A lot of people say that these two places may have the best beaches that you can see in the world but it is a matter of preference which one you would like to visit among the two.
Vacationing in Bali is known to be cheaper especially since there are a lot of inns there that will not require tourists to spend a lot of money. Fiji, on the other hand, may be more expensive but it is also cleaner and some say that it is also safer in most parts of Fiji. Remember that costs will vary a lot depending on your strategy.