For consistent tuning, less contaminants in the oil, and the longest possible life from a shock the plastic shock housings are better than aluminum.
Aluminum expands and contract with heat and cold much more than the plastic causing major changes in shock tuning especially if the car has been sitting in the sun.
The plastic body will not wear and leave contaminants in the oil like the aluminum shock bodies. Contaminants cause inconsistent tuning and increased wear on the shaft, seals and shock body.
Take the stock Traxxas shock, install the TiN rods and aluminum caps and they (the shock body) will last for years.
The aluminum shocks look kewl and the threaded bodies make dialing in the spring tension nice but when someone decides to make a comparable shock body from plastic it should take over.