Which experience is better for a mechanical engineer in UAE: Mechanical Maintenance or HVAC ?

HVAC is always better than maintenance.

I would say choose the one that interests you the most. I believe you will excel more on that field and gain higher expertise.If they are all going towards HVAC, go the opposite way.Less competition means higher success rate. 

HVAC ... is preferred

Mechanical Maintenance is better for feature growth. 

HVAC first, after ages of experience one will attain maintenance knowledge.

I would recommend to go for Maintenance first and then switch to HVAC in a couple of years later.

It will increase your confidence.

Both r important for mechanical engineer HVAC and mechanical

experience makes a man perfect, no experience is useless. out in the field all the engineering deciplines are overlapped in any field, construction for example includes civil mechanical and electrical and one way or the other even a little knowledge is helpful at different situations. 

As for my experience maintenance its a huge Umbrila , which its the main thing for mechanical engineer, but since my field its HVAC, i recommend to starts with the HVAC then by time you can find your self in the maintenance field because everything lead to maintenance.