Quoted from PanzerFreak:

It's not just bigger TV, and costs more. Also, do JJP pins really cost more when Stern is charging $7500, $9500, and in some cases $15k for comparable pins? Not really. Well except for GNR, what JJP did with GNR is pricing is disappointing.
If JJP continues with GNR pricing for all future titles please ignore everything I said below lol. Same thing if they raise Wonka prices to GNR levels.
To start the JJP quality difference can be seen in the build quality of the cabinet alone. The cabinets themselves are made better, the seams on them are cut better, the interior / underside is finished nicer, and heck the head is actually made out of wood versus 2/3 sheet metal. JJP is also still including traditional locks mechs for cabinet heads, a proper lockdown bar, and full support rails.
We can then move onto the features / toys, there are more of them on a JJP pin versus a vast majority of Sterns. On a JJP pin you will find multiple 3D molds that are detailed. Take the skull Axel mold on GNR, the spinning Gobster on Wonka, and especially Smaug on JJP (plus the gold piles and pillars). You will find nothing close to that on a Stern pin, not at the same level of detail or build quality. Example, LZ has a Zeppelin toy that is made of two half pieces of plastic that are held together by two exposed screw heads. The raptor tower on JP are thin plastics screwed onto a chunk of metal (again with exposed screw heads). This is inferior to say the 3D molded toys on Dialed In. Black Knight SOR again uses thin flat plastics on the catapult toy that are screwed into a chunk of metal. Some people may say you can buy after market mods to "fix" those areas but you shouldn't have to with a $6k - $9k+ pinball purchase of all things.
Finally there's the code work and JJP puts far more into their code work compared to Stern. A lot of Stern games are filled with "shoot this ramp" or "shoot this orbit" for X score with little mode choreography (unique music, sounds, animations, etc all tied to shots that work the entire playfield). All of those "super modes" that Stern loves to fill their pins with are copy and paste filler modes carried over from one game to the next. The Stern game that went against that mold the most was LOTR and we know where the programmer, Keith Johnson, ended up...JJP.