Telescope Reviews: Eyepieces for XT10i

The idea I outlined presumes that you would like to have eyepieces you can ALWAYS use, i.e. magnifications that won't be overwhelmed by poor seeing conditions.
With a 1200mm focal length, anything from 6.7mm to 8mm would be a great choice for a high power eyepiece. When I had a scope with a 1280mm focal length, I could ALWAYS use an 8.8mm, but the 6.7 was occasionally getting into "damaged by poor seeing" area.

Now, assuming your scope is well-collimated, and well cooled (on a 10", you WILL need a fan), then bad seeing is likely to pop up in that 150-180X range. Somewhere in that range, you'll start noticing that seeing is causing a lot of scintillation of the images.
So, lower in that range (150X), you're a little less likely to be bothered by poor seeing than in the upper part of that range (180X). It does depend a lot on where you view.

And today's high quality barlows really don't damage the image quality, though, as you can see, you'll push the magnification into the "seeing dependent" range very easily.

The GSO 2X, 2" ED barlow is a fine barlow, and it's usable for both sizes of eyepieces. And produces two magnifications for each eyepiece. Here's how [by the way, the Astrotech 2" 2X ED barlow is the same thing]:
For higher magnification (2X), insert the barlow in the focuser and the eyepiece into the barlow. Use the provided adapter for 1.25" eyepieces.
For lower magnification(1.5X), unthread the lens assembly from the barrel of the barlow and thread it directly to a 2" eyepiece. For 1.25" eyepieces, thread the barlow lens directly onto the 1.25"-2" adapter provided and use the eyepiece in the adapter. Be careful not to let the eyepiece barrel hit the lens, though. Pre-check the insertion depth of your eyepieces in the adapter and use a longer adapter if you are afraid your 1.25" eyepieces will hit the barlow lens.
As you can guess, the distance the lens of the barlow sits away from the eyepiece determines its magnification. If you pull the eyepiece slightly out of the barlow and re-tighten the setscrew and then refocus, you've just increased the magnification slightly. Having a lens that unthreads from the body of the barlow and which threads into eyepieces makes this a versatile barlow.
In that way, you can increase the magnification a bit when you want to "push the seeing" without going all the way into tiny exit pupil/poor seeing territory. That makes an 8mm eyepiece very practical because it can also be a 5.3mm and a 4mm for those very occasional times seeing allows the higher power.