I'm assuming you're on PC.
I personally love mGBA, but it can be very difficult to get away from VBA. Many hacks rely on VBA-specific quirks (or, less charitably, bugs); for example, some of the RTC implementations for Fire Red will only work in VBA (or its descendants) and are totally broken on actual hardware (via flash cards) or more accurate emulators (such as mGBA).
Whatever you do though, avoid classic VBA, because it has myriad easily exploited security vulnerabilities. I would generally recommend using VBA-M to play hacks and mGBA to play unmodified games.
However, if you are making a ROM hack, I highly suggest you do the bulk of your play-testing on mGBA. mGBA's emulation of the GBA hardware is far more accurate; if it works in mGBA chances are very high that it will work on actual hardware /and/ VBA-M.