The mirror lens is f/10 (and M42 screw mount), the Sigma is going to be f/6.3 - double that for each 2x TC.
Mitch reports his experience using a 1325mm telescope (on an APS-C sensor): the frame is filled, the moon moves out of frame in 10 seconds, and the result is soft; so you want a fast shutter.
The mirror alone will be brighter and likely not as sharp as the Sigma with a quality Teleconverter, which is expected to be sharper but darker.
The mirror has one focal length while the TC will fit all your lenses (doubling your portfolio). I know what I'd buy (the TC) but the mirror is low cost (plus adapter, assuming you don't have an M42 screw mount camera).
With 2 TCs you'll need an equatorial mount to race the moon and keep the shutter open.
It's a question of how many photos of the moon you want and how good you want them to be, for how much money; you can download $1M photos of the moon for free.
If you want photos of all the planets and stars a telescope with a computerized mount will set you back about a thou (depending upon what you want). Enjoy downloading from 10's of thousands of websites while you decide how much time and money to invest, and for what result.