I have a question... What is the highest magnification you can go on a usual basis with a 6 inch telescope? I know that they are marketed as having useful maximum magnification 300x but due to atmospheric turbulence etc. you can't go that high.
I'm planing on buying a 6 inch f/8 telescope and an eyepiece to complement the 25mm SPL eyepiece that already comes with the telescope and I want an eyepiece that I can use most of the time for higher magnification.
As others have suggested, for someone wanting a single high magnification eyepiece to go with a 6-inch f/8 telescope that comes with a single 25mm (about 50x) eyepiece; 150x seems to be about right.
But ideally, one ought to have a wide variety of available magnifications. Different "targets" react differently to being magnified. Sky conditions vary. Different 6-inch telescopes are made to different quality levels. And yes, even different observers (with all other things being equal) may prefer using different magnifications
With that being said, a decent-quality 6-inch telescope, under good enough conditions, can be usefully used at 300x or even a bit higher; but such magnifications aren't likely to be of use very often. -- thus the 150x suggestion.
When I go out with a 6-inch telescope (mine is f/6.5, not that it matters all that much), I may take out with me a single eyepiece when I know in advance what I'll be observing, etc. But when I go out without a specific observation in mind, I take out with me a variety of eyepieces The variety isn't always the same, but it's always a variety. Based on what's currently in my "take-out-box":
I'll often choose one 2-inch wide-field eyepiece that provides the widest true field of view I can achieve with the specific telescope. For me, this ends up being a 26x eyepiece. Then I'll likely choose a more "reasonable" (for me) wide-field, 2-inch eyepiece of 31x. Next come the 1.25-inch eyepieces. Again, I start with an eyepiece that provides the widest true field possible in the 1.25-inch format. This time, it's a specific 40x eyepiece. Then I go with a 'medium' magnification eyepiece of 110x. Next up are the higher magnifications -- 165x, 200x, and 330x. That last one is just in case the object, conditions, etc. all work out in such a manner that 330x might be practical and useful. Most of the time it won't be. 200x is often useful for me and my telescope; but I still think that 150x is the better choice if you can only get one additional eyepiece for high magnification use. 150x is a more "flexible" high magnification eyepiece -- useful for a wider variety of objects.
But we all have to start out somewhere. A 150x eyepiece provides a useful, reasonably high magnification for a 6-inch telescope that ought to be useful quite often. Higher magnifications (depending on the telescope's quality, etc.) may be useful at times; but much of the time they may not be. Circumstances (especially with atmospheric contidions) are not always the same and will not always support the use of higher magnifications.