Up to you. Bixby has really gotten quite good and is highly integrated into the phone's OS to control the settings. If that's important to you go with Bixby. Gemini can link to all the Google apps, like Google Home for instance. So if controlling home devices by voice is important then this is a good option.
Try each out.
For me, long press the side button is Gemini and double press volume up button is Bixby. Best of both worlds. ๐
