On the one hand, the $140 fee for the GRE v/s the $250 for the GMAT would tend to tip the balance in favour of the GRE.
On the other hand, IIRC, the GRE involves memorizing tons of words that may not be everybody's cup of tea.
On the 3rd hand (

) the GRE IS easier than the GMAT.
If more and more schools adopt the GRE acceptance, the GMAT could lose numbers.