If they continue with a HAL-operated HEX I would imagine it will go to Paddington mainline unless it was continuing on to the East. After all, once it's in the tunnel it can't turn unless they are building extra sidings and platforms to allow it to turn. The central section of Crossrail will be handling 24 trains each hour at peak leaving little time to allow turning etc (a bit like LHR's runways!) Apparently there are issues over trying to balance the service across the four arms of Crossrail as some will be busier than others! Also some trains from the East might need to terminate at Paddington to allow the HEX to continue to operate as a Paddington-originating express.

Given the times quoted, it will largely depend on fare difference as to whether I plump for HEX + tube, Crossrail + tube or just tube to where I need to be in London (Southish....)