I guess its down to personal choice..
RAM (metal) or RAP (plastic) parts are offered in the UK.
For handlebar mounts, the RAM metal uses a U-bolt bracket with a zinc or stainless steel U - bolt and I suggest using this with the plastic shim (RAP-319) when attaching to a cross bar or mirror arm. If you are just attaching onto the handlebar then you can choose and have plastic with a RAP-B-149 and the only difference is the U-bolt style.
The plastic is a durable polycarbonate base and the arm is glass reinforced nylon - so its tough and will do the job.
So as I say .. its personal choice.
RAM-man