first up i'd like to offer kyou a huge welcome!
ok now t you r question.... i'd go the 8mm cause it gives you extra eyerrelief and would be more comfortable to use.
in my f6 8" the max i tend to use is a 7.5mm. this gives me 160x. in average to above average this is as high as i'd go. i have a 6.5mm which gives me about 182x and i use this only when the seeing is good.... under exeptional seeing i'd barlow my 10.5 (@ 228x) and this is about the max i'd go on a planet.... double stars are another thing entirely.