Have a look at
http://www.televue.com/engine/TV3b_page.asp?id=154
and specifically
http://www.televue.com/engine/TV3b_p...n=Advice&id=80
Is your 8" f6 Newtonian on an EQ mount or is it a Dob; i.e. doies it have tracking? If you have tracking then you are likely to observe at the centre of any eyepiece so edge of field aberrations are not a concern. If you are observing without tracking then you may consider a wider field eyepiece with good correction and this is where you either have to try a lot before you get something you like or pay up front for a quality brand designed for any telescope, typically down to f4.
Your question though was specifically for planetary eyepieces and usually on axis sharpness is most important for these as typically planets are observed with tracking as far more detail can be noticed as you are not distracted by having to nudge the 'scope as the object moves to the edge of field.
You also have to ask yourself what eye relief do you prefer. If you require eyeglasses when observing then you need 20mm eye relief (and these are very comfortable too for non-eyeglass wearers). If you are fine with putting your eye close to the eyepiece then a Plossl or Ortho is fine and will perfom very well in a Newtonian with excellent on-axis sharpnes and reasonable off axis performance. Buy the best Plossl or Ortho you can afford and for your equipment a Tele Vue 8mm Plossl at $129 from Bintel would be ideal with change to spare.
Another option and perhaps preferable would be for you to buy a Tele Vue 15mm Plossl and then a Tele Vue 2x Barlow (unless your current Barlow is of good quality long design. (Shorty Plus is supposed to be good but a standard short Barlow has too many abberations and vignetting is introduced with most eyepiece combinations but as I have not used either type I'll recommend what I have and a 2x 1.25" Tele Vue is a very fine Barlow). The 15mm/2x combo will give you a 7.5mm eyepiece and 160x and 10mm eye relief which is comfortable for a 50 deg. eyepiece and non-eyeglass wearers. Later you could complement it with an 11mm PLossl for 109x (and 218x with Barlow) and then later an 8mm which will give 150x (and 300x with Barlow).
Note I mentioned exit pupil in my previous post and this refers to the diameter of the cone of light exiting the eyepiece and is directly related to the brightness of the image that you see. It can be calculated by diving f ratio of telescope into focal length of eyepiece, e.g. 8mm/ f6 = 1.3mm exit pupil. It can also be calculated by dividing the magnification into the telescope aperture, e.g. 200mm/150x = 1.3mm exit pupil.
Typically your magnification range will be from 0.5mm exit pupil to 7mm exit pupil (maximum eye iris opening, typically at 7mm). This equates to a 3mm eyepiece to 42mm eyepiece at f6 which further equates to 400x to 28x with your 1200mm focal length telescope. At the smaller exit pupils eye floaters become visible and annoying but it is observer specific as to how much of a problem it is. I can observe down to 0.4mm exit pupil without too much problem below that the image simply is too dim for my liking especially for low contrast objects like planets although I can go lower for high contrast objects such as double stars (i.e. more power).
I mentioned the 5,6,7mm in my previous post and was indirectly referring to the TMB currently for sale as indicated by another member. However as I have not used these eyepieces I cannot comment on their quality and suitability for your telescope but they do represent excellent value and would more than likely be fine.
I don't think you should worry too much about what eyepieces suit what telescope simply because most manufacturers wont tell you or they only tell you half the facts and in most cases it really only matters the most for low power work. Just buy the best that you can afford even if you only buy one at a time. If you buy a reputable brand you wont be disappointed and its a keeper for life and you never know when you may end up with another telescope you can also use them on.
See also
http://www.televue.com/engine/TV3b_page.asp?id=214 for some specifications I mentioned.
Whatever you choose, enjoy!