Vertex will give you width! ...which is really important if you will be using your car in any stationary traffic-scenario, like at an accident. But they don´t give you the output to sufficiently grab the attention of oncoming traffic (from the rear) during bright daylight.
The rect14s will give you the output needed, but not the width if placed as suggested.
I could suggest putting better HAWs in, but maybe you´ve got these two products already. In that case: put the rect14s as suggested and mount the vertexes under your sideview mirrors to give you wide 360 degree coverage.