The 6X single lens 25mm (1") size is the most popular in general & as a promotional magnifying glass with measuring scale since it has room for printing full logos.
The 6X double lens 25mm (1") version has better color correction & is useful for printers who need to accurately measure color dot separation. Smaller logos can be laser etched
The 8X model has double lenses. Their higher magnification makes it good for inspecting coins and reading microprinting on checks. The double lenses give good color dot separation which is useful for examining stamps & authenticating paper currency. These can be laser etched with smaller logos.
The small plastic linen testers are ideal for product or service promotion since the vertical section has room for print or decal logos.