Your theoretical power for the 6" is 360, your practical power is 240, which is a 5mm ep. I encounter no probs on my 6" dob with a 5mm ep and a 4mm (300x) ep unless the atmospheric conditions are really bad, then detail suffers. As an example, with poor "seeing", the planet looks like your looking through boiling water. On typical nights the 7mm, 5mm or 4mm is what I use on planets. The bigger the aperture, the more you magnify poor atmospheric condtions. The 6" is considered to be the entry level "serious scope". On most nights, it will not disappoint you. Star clusters I find on mine are best viewed with a 25mm
Here's how you do the math for mag. on ep's:
focal length of your scope (this is yours) 1200
Divide by focal length of eye piece, e.g. 10mm = 240 mag.
To work out the max. power for your scope, multiply 60 per inch of aperture (60 x 6) this is 360 for your scope.
Here is a link that explains a bit about aperture and magnification:
http://www.seqas.org/
Regarding mid range eyepieces, I like the Celestron and Orion Expanse for their contrast and sharpness.