The cutoff point between BD's and giant planets is a real hazy one. There is a size definition of anything including and over 13 Jupiter masses. At this size, a body can begin to fuse deuterium in its core. It can also fuse lithium if the core temp is higher than 7 million K. However, then you also have to take into account how the object forms, and this is where things really get blurry. Reason being that it's quite possible for very large planets to form in the same manner as stars do, i.e. gravitational collapse of a pre-existing mass of gas (the "top down" approach), as well as from both methods of the "bottom up" approach (core accretion via planetesimals and slow gas accumulation, or core accretion via large, low density cores and rapid gas accumulation). It depends on time and the characteristics of the accretion disk they form out of. Accretion disks around most stars don't last for very long...for a star like the Sun, around 10-15 million years. So it doesn't give the planets much time to build up. A Jupiter sized world really only has about 2-5 million years to reach its full size before a substantial amount of gaseous material is lost during the star's T Tauri phase. For terrestrial planets, which take longer to grow (especially for Earth size planets and larger) than the giants, they can carry on longer due to the presence of larger sized dust grain material and bigger objects (such as asteroids etc) which are still hanging around after the T Tauri phase.
I think the best cutoff point for distinguishing a planet from a brown dwarf would be this...1. Top down development via gravitational collapse,
and 2. Fusion of low temperature elements such as deuterium and lithium. That would probably put the cutoff point in a range between 13-18 Jupiter masses. If the object still formed much like a star but is below the mass cutoff/no fusion, then it's a planet. If it's heavier than the cutoff and is fusing deuterium,
and apparently formed like a normal gas giant planet (bottom up approach), then it should be classed as a "superplanet". There are a few examples of objects like this, e.g
CoRoT 3b,
XO-3b. Both these objects are very dense and probably have substantial rocky cores, or a lot of rocky material in their cores, and they're both substantially larger than Jupiter.
So, this is where we have to be careful about how we define what a planet is and how it forms. It can get rather murky!!.