Hmm, I’m torn here. Most useful for ME? TRX. Most useful for some of my clients (such as elderly, overweight, or inexperienced), BOSU. Most useful for some of my athletes and advanced clients, TRX.
So my answer would be “it depends.” Based on the client I’m training and the goals of the particular training (for example, I use the BOSU a lot even with the athletes in some balance and plyometric work) each has its utility (I know that’s not the answer you want – which was to choose ONE – but it’s the most honest answer that I can give you 🙂