Generally there are upfront savings to be had when subrogating the developers mortgage:
Notary and Registry fees are halved
There is no stamp duty to pay on the mortgage amount
There is usually no bank opening fee imposed
There is no valuation fee to pay
Therefore dependant on your mortgage amount, these savings can add up. If you weigh up these savings vs long term cost i.e. is there a high interest rate involved? and should the product criteria fit your requirements, then it can be the best option. Also check to see if there are any hefty comulsory life assurance policies involved!
On the downside developer mortgages are inflexible and come as a percentage of the purchase price rather than the valuation.