Seaguar Invis X is my preference. Just be careful with knots. You must be sure to wet the line well when sinching a knowt with any fluoro or you will hate it.

As far as visibility goes it (fluoro) has a light refraction closer to water which makes it LESS visible than mono. Fluoro is a stiffer line (less stretch) which in turn makes it more sensitive. This stiffness has its advantages and disadvantages as well.

No matter what line type you use the smaller the diameter you can get by with and retain the tensile strength you need for the application you are using the better for two reasons.
One is the obvious sight thing the smaller the harder to see. But dont forget that our fish FEEL vibrations as much as see our lines, therefore I suspect a small line may be felt less as well. Our lines in the water would be no different than a guitar string.