This is one of the reasons i love my auto tints, 9 to 12 or 13 depending at the push of a button. :)
Then its just if you like the color of the auto tint, Speedglasses have a green hue much like the standard green filter plates and my jackson has for lack of a better description, a "gold" view to it. It looks like the gold plated filter plates but with just a hint more yellow.
I prefer the jackson, but my adflow is hooked up to a speedglass so i use both.
Not too sure about other electric lenses, most of the cheap ones looked green like the speedglass.
-Clif
Good rule of thumb is that when you get done welding, you shouldn't see a bright spot in your vision. If you do, I'd suggest that you got to a higher shade till you can't see it anymore. Everybodys eyes are a lil more/less sensitive than others so it depends on the person too.
Also depends upon how you hold the various guns, rods, etc. In other words, are you actually looking at the arc, or without seeing much arc are watching the puddle? If you are somewhat shading the arc and are trying to see where you are in relation to the joint as well as watching the puddle formation, you will not need as dark a lens.
I also vary my lens depending on outside light sources. But, generally speaking, I use a 10 and occasionally an 11. GTAW, GMAW, FCAW, and SMAW. Will also depend upon the amps being used. Spray arc GMAW on clean, bright steel will really make you squint, sometimes a 12 is handy.
My two tin pennies worth, Have a Great Day, Brent