With legacy carriers, seniority is everything and if you want a long term career with a particular airline then get on the list as soon as you can. If you want to go contract flying then get your upgrade and a couple of thousand hours in the left seat before moving on.
Expat airlines such as Emirates and Qatar will take direct entry Captains when needed however it may be easier to join as an F/O and get used to the operation before they upgrade you. Failure rates for DECs can be higher as the training department need to be hard as you are an unknown quantity. You would be less stressed with a couple of years in the right seat getting used to the routes and SOPs, and then undergoing their own training for the upgrade.