I feel that this topic is being beaten to death. I personally own both the 12000 and the 15000. They are both beautiful machines and do wonderful sewing and embroidery. However, I find the 15000 to be more intuitive and it does have much more than the 12000. What is most important is:
How much do you want to spend?
Do you want the latest technology?
What are your needs? Maybe the 9900 will suit what you really need without the large financial burden.
The 15000 has "micro" embroidery capability - Do you want that? The 9900 also has that feature - it is one of the new features in Janome embroidery - if I understood one dealers explanation correctly.
I wanted it all, but decided to keep my 12000 as the hoops are interchangeable, so I could start something on one machine and finish it on the other, if I wanted to do that, or hoop a second project and have it ready to go with the first is complete. I also wasn't going to starve to death or not pay my bills if I had 2 fancy machines in my sewing room - I'm collector.
I suggest that anyone wanting a new machine, should locate a dealer - take in some fabrics and do a test drive and then make their own decision. After all you are the one that will be using the machine and no matter how many opinions come up on a forum, they will never agree and in the end you still need to make your own informed decision.
Good luck with your decision - I'm sure that whatever you decide you will be happy.
Cheryl - Saskatoon