Get the most aperture you can afford. Any good Dob can be pushed
gently enough to follow a planet at it's highest practical power. I have
a six-inch refractor with a motor drive and a ten-inch reflector that is
hand driven and I would choose the ten inch any day over the six for
planetary viewing. Since I got the ten inch, I only use the six for lunar
or solar. Anyway, if the astronomy bug bites, it won't be your last
scope.
Larry Brown
http://home.fuse.net/astronomy