Digicel is the same price as Flow. Be sure your phone is "unlocked" before you go to BVI. I took two phones, an old Samsung Galaxy Note 3 on AT&T and an iPhone 5S on Verizon. Turned out both were "locked" to the carrier. It is easy to get them unlocked in the states, easy in BVI too but you need to call a US number and log onto a webpage to accomplish this.
Digicel has an app that you use to top up minutes, don't know about flow. A learning about Digicel is that you must have some activity on the phone every two months. If you don't travel to BVI every two months then you must do something else. The Digicel tech told me to put $3 credit on the phone every two months which I do via the app. Also, you get a one-minute warning when your minutes are going to run out on a phone call. There is no warning on data. When the phone times out you have a brick. You cannot get into the app until you find a hot spot to top up.
The Digicel folks were super friendly and I really like having a local number. My wife has AT&T and does the $10 a day thing. Works like a charm. Makes her feel secure that our kids can call us without having to look for a number they haven't memorized.