Answer:
GTX comes with a hemi standard. Roadrunner does not.
Actually the standard engine for the GTX was the 440-4bbl, the 426 HEMI was an option, throughout it's entire production run as a stand-alone model from 67-71'. The 440 6bbl was also available as an option in 70-71'. The GTX was considered the "Gentleman's muscle car" as it had a higher trim level and price tag.
The Road Runner in those years was initially a bare-bones/big motor version of the GTX, 383HP being the standard engine in 68', the 426 Hemi and, from 69 1/2', the 440 6bbl were optional up until 71'. It was marketed toward a younger crowd. For 72-74', with the GTX discontinued as a stand-model, Road Runners ordered with the optional 440HP engine were called a "Road Runner GTX".