Hey mate after having this same debate over my friends engine the GPX is without doubt the better engine. It was chosen as a worthy upgrade after his Lifan 140 began to have a whole array f problems from gears to clutch. After chatting to the guys at motorbike city they said problems like these are common with the lifan 140's and strongly recommended that we should not even consider getting another.
Now onto the GPX 125cc, the engine packs a punch, after riding around my mates paddok the hting just wants to go and keep going... another few good features is the easily accessible oil filter which can be asily removed for cleaning and servicing also the enginew looks beast.
In conlcusion I highly recommend the GPX 125 over the 140cc Lifan, both are aroundf the same price mark but the GPX offers better reliability, punchier power (maximised through carby choice, oko recommended) and looks a ton better with a few handy featuers
Hope this helps
Regards
Carlton