Is it possible, and/or worth it, to swap my 5 speed to a 6speed in my 2008 Miata?

Mazda did have a 6 speed which would be a nice way to upgrade your car. I would check Miata forums for more information on the swap.

If you can sell your Miata to get the one you want that would be cheaper. Because drivetrain swaps are expensive up to 3000 most

You could source all the parts and do the swap, and it would be a fun project. However your better off selling your car outright and just purchasing the car you want with the options you want. You have to figure getting all the parts plus your cars down time and if you are going ti swap it yourself or pay someone. If you do decide to do it yourself you could try finding a wrecked version of the setup you need and purchasing the whole car, which would be a great refference and provide everypart needed. then just sell off the parts car when your done. Good Luck and lets us know what you decide and how it works out.