Differences include:ARH vs SDLLow grade vs high gradeComplex...

Differences include:

ARH vs SDL
Low grade vs high grade
Complex beneficiation process vs simple proven mining/crushing
"right to mine" vs proper tenure
slurry pipeline vs rail (plus distance to port)
non-credible vs credible management/Board
single majority shareholder vs proper corporate governance

As always not all points are positive for either ARH or SDL. Management is a huge factor - SDL has (and had) a far more credible Board. ARH has a majority shareholder who has never developed a mine. There is a question as to whether the current board is looking after minority shareholder's interests.

Whilst some infrastructure can be shared, it has not been disclosed what this encompasses and how much extra stuff will need to be built.

My uninformed opinion is that there is a very good reason for the discrepancies in market cap.