One factor is whether you might want a backup camera. If so, definitely go for a double DIN with a screen. For my wife's 325xi, I tried to use a replacement rearview mirror with an embedded screen for the backup cam. But none of these mirrors with integrated screens have any method of manual or auto dimming for rear headlight glare. That was just awful with today's extremely bright LEDs.
I was using a Bluebus and it is awesome at its job. In addition to the great audio it adds some nice features (like auto unlocking all the doors on engine turn off if you want to enable it) and the dev is very active. But ultimately we needed a screen for the cam so I took out the horrible mirror, went back to stock rearview and added a double din, Sony AX3200. That got me BT as well as Android Auto and Carplay.
So if you don't need a cam, the Bluebus is ideal. If you do want a cam or AA/CP then double DIN with a nice screen. I'll plug Enfig car stereo here if you do go with a new HU. They have all of the adapters to make it plug and play, integrate with the steering wheel, and the dash kit/relocation brackets are nice.