I don't think STX selling a gas field is at all likely. Just...

I don't think STX selling a gas field is at all likely. Just saying December is a long way off adn their could be "black swan" events.
If you think value Mongolia at zero then TPD shareprice will not outperform STX so why not swap out now and avoid any black swan events?
If you think STX will rise faster than the Mongolian contribution then swap out now.
If you think the Mongolian value will rise faster than STX (so the TPD shareprice is (STX/2) + % then keep them.

The difference between 19.5c and 21c is the time/risk cost of waiting until December.