I have the FA*300 f2.8 and it works very fine with the Pentax TC. I also have had the Sigma TC, it doesn't work with the Pentax 300. What is important is that the Pentax TC converts the focal length to 420 mm, this is essential for a good image stabilisation. The Sigma doesn't, and as it transmit the focal length of 300 mm, the image stabilisation will not be efficient.
I haven't had the opportunity to work with the sigma, for me that is using it a lot for birding, my choice was driven by the faster AF of the Pentax, plus the good reputation of this lens. The faster AF is related to the focus limiter that prevent the lens to explore all the focusing range if focus is lost.
The Pentax is build like a tank, even if the silver finish is very prone to scratches.
As I own 2 copies of the FA*300 and want to sell one, cause I'm on the low side of money after paying all the taxes that the french government has introduced this year - may be I'm not objective if I suggest you to choose the Pentax
. But nevertheless, when I had to choose a F2.8 300 I went to the Pentax without regret.