At the least, buy this.
The reason it is the least, is lint will still get trapped in the edge that goes into the flexible part. Joints should be placed so the moving air flows over the joint, not into the joint. The reducer you have at first will restrict the flow, and still have the joint facing in the wrong direction, although you would not need to cut the flexible pipe.
The corrugated ends of the piece I linked to will fit inside each end of the vent pipe and the flexible duct, and minimizes restriction. It would be best if the flexible pipe was reduced in the same way, then the air would flow over the joint the proper way. There is a tool to do that reducing, any duct man worth his salt has this.