A la carte wedding photography has always been an option for wedding couples in Singapore. Unfortunately, due to high land costs which translates into exorbitant rentals, the number of studios offering such services are shrinking.
The main attraction of going for an a la carte photography package is the "quality" of the photography. I put the word quality in quotation marks as quality here refers more to style than technical skills. Photographers at one-stop boutiques are employees and as such, are either constrained by company procedures what shots to take, or are unmotivated to do extra. A la carte studios on the other hand are owner operated and hence they are willing to stretch themselves for their own artistic visions.
As usual, I cannot say which is better as it is a personal choice. What I can however say is that quality is similar for both the a la carte studio and the one-stop boutique. The extra you are paying for an a la carte service is not for quality per se, but for the option of having photos which are different.