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A 6" f8 is an excellent telescope that will last you a lifetime. The longer focal ratio does mean that you can use less expensive eyepieces not specifically designed for shorter focal ratios since as off axis abberations will not be as pronounced. However premium eyepieces have other attributes than just sharp stars to the edge and would be worthwhile additions.
An 8" mirror has almost twice the light gathering capability as a 6", (8x8=64, 6x6=36, 64/36=1.78). Many DSO's are faint and every bit of light is welcomed. An 8" f6 will likely have a 2" focuser whereas a 6" f8 may only have 1.25" (2" can be added though not always easily). A 2" focuser will allow much wider true fields with long focal length 2" eyepieces, but with an f6 telescope these eyepieces will need to be highly corrected ones such as Tele Vue or you will see abberations towards the edge of field.
The maximum true field for a 1200mm telescope with 1.25" focuser is 1.3deg whereas for a 2" focuser is 2.2deg.
A 6" will show you all the planets in the Solar System (even Pluto is within reach), all the Messier objects and many galaxies, globular clusters, open clusters, planetary nebula and even one Quasar, 3C 273. An 8" will show the same a little brighter and a few fainter galaxies not within reach of the 6". Note galaxies not within reach with the 6" but visible in the 8" are going to be very faint.
If you buy a 6" now your next telescope if any should be a 10" for 3x the brightness increase. If you get an 8" now then to see a significant increase in brightness your next telescope should be at least a 12".
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