Without more context, it's tough to give a definitive "best" answer. However, here's a breakdown of considerations for using Greenfield (Flexible Metal Conduit - FMC) and EMT (Electrical Metallic Tubing - rigid conduit) for low voltage wiring in a wall to a switch, along with my general recommendation

Factors to Consider:
Code Requirements

Local electrical codes are paramount. They may dictate which conduit types are allowed or required in specific applications. Always consult your local code.

Type of Low Voltage Wiring

What kind of low voltage wiring is it? (e.g., thermostat, doorbell, network cable, speaker wire, etc.). Some types are more sensitive to interference and might benefit from the shielding of EMT.

Environment

Is the wall in a dry, damp, or wet location? This affects the suitable conduit types.

Accessibility and Installation

How easy is it to run conduit in the wall? Are there tight bends or obstructions? How familiar are you with working with each type of conduit?

Shielding

EMT provides better shielding from electromagnetic interference (EMI) and radio frequency interference (RFI) than FMC. This is rarely a concern for DC systems

Cost

EMT is generally cheaper than Greenfield

Greenfield (FMC) - Flexible Metal Conduit



Pros



Flexibility

Easier to navigate tight spaces and bends in walls.

Easier Installation

Can be cut and fitted relatively easily, especially in retrofit situations.

Less Cutting

Does not require threads

Cons



Less Protection

Offers less physical protection to the wires than EMT.

Not as Durable

More susceptible to damage if crushed or kinked.

Grounding

Proper grounding is crucial. Usually requires a separate grounding conductor inside the conduit.

Not for Wet Locations

Not suitable for wet or damp locations unless specifically rated.

EMT (Electrical Metallic Tubing) - Rigid Conduit



Pros



Excellent Protection

Provides robust physical protection to the wires.

Good Grounding

Can serve as an equipment grounding conductor if properly installed.

Shielding

Provides excellent EMI/RFI shielding.

More Durable

Resistant to crushing and damage.

Cons



Rigid

Difficult to bend and navigate tight spaces. Bending requires specialized tools (conduit bender).

More Difficult Installation

Requires cutting, threading (sometimes), and specialized fittings.

More Expensive Installation

Higher labor costs due to increased difficulty.

General Recommendation

In most residential low-voltage scenarios within a wall,

Greenfield (FMC) is often the more practical choice if permitted by local code. Why? The flexibility and ease of installation usually outweigh the slightly reduced protection, especially for low-voltage wiring that isn't carrying significant current or particularly sensitive to interference. However

If the wiring is in a location where it's likely to be physically damaged, or if the local code requires it, then EMT would be a better choice.

Important Considerations for Low Voltage Applications



Bundling

Be mindful of bundling low-voltage wires with high-voltage (120V/240V) wires. Maintain separation to avoid interference and safety hazards. Different voltage classes should never run in the same raceway.

Low Voltage Codes

Refer to your local electrical codes for requirements specifically related to low-voltage wiring installations.

Grounding

Ensure proper grounding of the conduit, regardless of the type used.

Professional Advice

If you're unsure, consult a qualified electrician.

In summary: Check your local codes, assess the environment, consider the type of low voltage wiring, and weigh the pros and cons of each conduit type. For most typical residential low-voltage switch installations, Greenfield is a reasonable choice, but always prioritize safety and code compliance. Flag for review