Things to think about with the Jazz: although it has 12" to the right of the needle, the space from the bed to the bottom of the arm is much lower than the space in either the Juki or the Brother PQ1500s, so it actually doesn't have any more room to offer for pushing a large quilt through the machine. The entire harp space, side to side and top to bottom can make a difference in ease of maneuvering a quilt when FMQ. Also, the head of the Jazz is much wider in order to allow for movement of the needle while forming the extra stitches which means that the visibility around the needle won't be as good as on the Juki or Brother PQ1500s-- this won't matter if you just want to do straight line quilting using a walking foot or regular foot, but it will be important if you are interested in FMQ. Both the Juki and the Brother can be set up to stop with the needle down which is useful (though not necessary) when FMQ. The Jazz does not have this option. Several people have reported that when piecing with the Jazz, they had a problem with it not wanting to go over seams. That was reported in at least one review here on the board.
If you go for the Jazz, be sure to take a lot of samples with you to make sure it will do everything you want it to do. I'd take a largish quilt to see what it's like putting it through the harp. I'd also take some piecing that has lots of intersecting seams to make sure it will feed over the thickness of the seams. If you have been FMQ on another machine, then take some sandwiches and see what it's like as far as visibility goes for FMQ.
Rob