There are lots of options when deciding which manufacturing method to use. Each method however is dependent on a few factors.
The reason why this is important is because every manufacturing method vary in costs.
For example, parts are generally cheaper to make using injection molds opposed to casting but you need to also take into account the cost of the tooling. The cost of the tooling for injection mold is more expensive than the cast tooling so dependent on how much initial cost you have this is a very important factor.
You need to be a little sensible on deciding on the material to use. Think about the application, think about the weight and most of all thing about how long the part needs to work for.
Don’t choose a manufacturing method that produces a invincible, wonderful looking and expensive part if people are going to store it in a cupboard.
Remember there are tricks to change the strength of a part by adding ribs for instance or even making the material stronger by adding another material to the mixture. Typically adding glass to a plastic part will increase its strength but be careful as the more glass you add the more brittle the part. Same applies to alloys in metals during casting.
Your designer will have a vast knowledge (or they should) on this matter. I suggest you discuss this with them if you cant decide on a manufacturing method to use.