Go loooong! f/6-f/8 for newt, f/12-f/15 for a refractor. Saves you a lot of headaches with coma in newts, less chromatic aberration in refractors, much more forgiving of collimation errors, and can use smaller secondaries with newts. Also easier to cut out stray light with longer f/ratios which is critical to increased planetary contrast.
Also with planetary viewing you'll be using mainly higher powers and having longer f/ratios will save you from fussing around with barlows and short-eye-relief 'peep-hole' eyepieces.
Equatorial tracking mount is nice, but certainly not required. I track pretty easily at 400-600x with my 10" f/8 planetary dob.
I've found that an 8" to 10" newtonian is my instrument of choice for planetary viewing. If I had super steady skies I might consider something larger. For a refractor, 6" seems to be the practical size/ price limitation - they start getting really large/ heavy and expensive after that. The standard rule is the longer the focal ratio, the better. For newtonians, the longer focal ratio has the advantage of a smaller secondary mirror which reduces the effect of the central obstruction. For refractors a longer focal ratio means less chromatic abberation. For an achromatic doublet in larger sizes, f-12 is generally considered the minimum. A longer focal ratio also has the advantage of being easier on eyepieces and not requiring the use of a barlow in many cases. For newtonians f-6 is a good starting point. For achromatic refractors 4" and smaller f-10 and above. For lager achromatic refractors f-12 and above unless a chromacorr is used.
The design is less important than making sure you get excellent optics. The advantage to the longer focal lengths, as others have mentioned, is the ability to get high powers more easily. In reality, though, there are modern eyepieces of very short focal lengths, so this is less of an issue than it used to be. I feel that tracking, whether it is an equatorial mount, a computer drived altazimuth, or a Dob on an equatorial platform, is highly desireable for serious planetary observing. I used a "hand driven" Dob for years - and I would not go back to that era. The driven scope is just so much better for prolonged viewing and attempts to see subtle details, IMHO.