Welcome to the club.
I am certainly biased as an owner of a WSC, but I also sell Kamado Joe units and am very familiar with them. Both are excellent cookers, but each does have its pluses and minuses.
What I love about the Summit:
- Very easy to move around (doesn't weigh a ton)
- Extremely adaptable to whatever you are cooking and can swap from one setup to the next easily and mid-cook. eg Smoking > Searing without a long wait
- Much more durable overall. Ceramics can obviously crack (both inner and outer walls, as well as the D plates). Don't have to baby the exterior or worry about the cooker interior absorbing unwanted chemicals or smells.
- Much easier to keep clean IMO. Cant use a razor scraper or steel wool on a Kamado!
- Easier charcoal management, ie adding fuel mid-cook can be a PITA on a ceramic unit just due to all the hardware/racks/grates you need to work around on the inside. Piece of cake on the Summit.
- Less time required to be up and cooking with the gas-assist.
Kamado Joe:
- Extremely efficient. Uses even less charcoal than the Summit will due to thermal retention.
- Lots of cool (albeit expensive) accessories available right from Kamado (DoJoe, JoeTisserie etc..)
- Has more of "wow" factor in terms of visual appeal
A big plus about the Kamado Joe over other ceramic brands is that they do include most of the essential pieces like deflector plates etc outta the box. Many other ceramic manufacturers make you purchase it all separate and it adds up quick ($200-$400 extra in odds and ends).
Unless you plan on doing a LOT of rotisserie cooking I wouldn't make that the deciding factor between the two. I would base your decision more on the style of cooking that best suits your needs. Will it mainly be "for special occasions" cooking for long haul low-n-slow (Thanksgiving/Christmas etc)? If so the Kamado might be the better choice. Otherwise if you want more of a workhorse that will take almost anything you throw at it, any day of the week, in any weather then the Summit is an excellent choice.
The new Big Joe III (not II) also now includes their new "slo'roller" technology and a deeper body to accommodate it. It also costs almost 50% more (at least here in Canada). Cool design but not quite as revolutionary as they make it out to be. It essentially works the same as the deflector in the Summit and simply forces the heat out around the outer edge so you get more of a rolling smoke inside the grill. You could rig up something in the BJ-II to more or less do the same thing and save hundreds of dollars.
I would highly recommend going out and seeing both units in person as getting your hands on one can make all the difference in the world. You may find you like the height of the Kamado vs the Summit or vice versa...
Hope this helps,
Matt