The indications, guidelines and factors as well as all the different variations for use of a "cold runner" versus a "hot runner" are going to vary, and would range beyond the scope and ability to answer the question.
You will have to examine your application and choose accordingly, but I will try to explain briefly some of the advantages to a hot runner solution. As I currently work for a hot runner company I see many parts that benefit from this application. Here are a few basic reasons to look into hot runners:
Less waste -which in turn reduces contamination
Controlled melt temperature - reduces warping of part as well as plastic degrading
Great gate vestige - Reduces the need for second operations
Lower injection pressures - allows for more cavities per mold
Shorter cycle times - improves production efficiency
An example of this would be a high cavitation part such as a small cap. A hot runner could be used for this part creating a good cosmetic gate, no waste, with a cycle time. A hot runner can be used for all thermoplastic resins, and yes even rigid PVC if done correctly.
I would suggest that you strongly consider the SPE training program, hold at the Chicago Hilton/Indian Lakes Resort, Bloomingdale, Ill. Topics include: mold design and building; plastic materials and processing; extrusion of engineered plastics; reheat stretch blow molding; injection molding troubleshooting; plastic component failure analysis; thermoplastic elastomers, thermoforming process technology, plastic part purchasing and quoting; screw design; snap/press fits and plastics welding; blow mold tooling and part design; rheology, biopolymers and other new technology. Another alternative to consider is having someone come to your plant to conduct training.