It really is down to a matter of preference. In terms of aesthetics, the slim bass bow quiver is more discreet and is partially hidden by the tailpiece, whereas the triangular topped quiver is fully visible beside the tailpiece. During use, there are a few differences the slim bass bow quiver has a smaller opening for landing the bass bow but it holds the bow better meaning less movement from the bow when it’s in the quiver.
The triangular topped double bass bow quiver has a much larger opening giving really easy landing of the bow, but this also allows the bow more freedom to move whilst in the quiver. The slim quiver is smaller in length at 36cm compared to the triangular topped quiver which is 45.5cm so it will fit smaller double basses better for younger bassists.