to be fair the average gross is just above 5000 per month in avion express for a FO (if you count 500 BH and including the loyalty bonus).
When I looked at the last commercial from wizzair on linkedin it looks a bit the same (for example in Italy gross salary for FO varies from 4100 to 6100).
I'd say it boils down to which kind of life you prefer as command upgrade is also 3000BH and two OPC in Avion Express