i agree with what most everyone else has said. don made an excellent comment about those moments when the seeing gets better- that's when the extra aperture will show the big advantage. but i agree with him about a big dob- although they can make great planetary scopes, the bulk and added cooldown time can be something to consider. i like DSO's, so i have a big dob. but if i we're only interested in the moon and planets, my primary scope would probably be a flocked 8-12" newtonian with a curved spider, small obstruction, fans, a good mirror, and an f/ratio slower than f/6.
Homebuilt 16" Truss Dob in Ohia Forest Observatory
(12' X 12' rolloff under Mag 7 skies)
8" Antares f/5 Newt
SV 80mm ED Nighthawk NG on M1 ALT/AZ
Nikon Prostaff 65mm spotter on Trekpod
Konusvue 20x80 binos
Homebuilt 80mm f/5 refractor
Mirador 60mm f/12 1960's refractor
Loc: Loomis, CA
I second what Erik said. There where times during the early summer when jupiter was at it's highest where my 12.5" Dob could be pushed to 350 to 400x without image break down. Then there were times when I was lucky to see any more detail than my small refractor would show ( both using 120X). The big Dob would beat any of my smaller scopes on DSOs on the same bad night.
Misty Ridge Observatory
Many scopes 70mm to 12.5", SCTs, refractors, and newts and a lot of other astro stuff.