2.8mm
Offers a wider field of view, and is typically the broadest field of view for most standard CCTV Cameras.
• Covers a large area very well
• Good all round use
• Longer Range Focus
Conclusion
In conclusion there's a notable difference between the two cameras, if you need to see further away the 5mm is the camera for you, or potentially an even larger lens. Whereas, if you want to cover a wide FOV then 2.8mm makes the most sense.