We are designing a web application. In the grid view list we have a add new link which can open a modal pop-up window or push down the content like an accordion type user interface.
Regarding usability of the application, which one is better?
Popping a modal means that the user must deal with the modal before continuing: complete its task or mouse to its close button and get rid of it.
Push downs don't require any additional action from the user. I can open up one of those suckers (or many of them) and just scroll past.
That's not to say that one is always better than the other. That depends on your context.
Use modal window if it's a multiple actions/screens flow (add link, name it, add tags … submit), use push-down if it's a single action operation (paste link, submit).
I say that in the 1st place you need to analyse how the users will use your application and what kind of devices will they use (Is is mostly desktop/mobile/tablet). Afterwards you need to explain a little bit better what is happening in that "Add new link" action:
You just need some more details and the answer will pop in your head :)
Also with pop ups specially modal ones, you can only work with one record at a time. With slider model, you can have multiple rows open and edit/update or at least have a “view” of editable/updatable fields.. or even can do multiple updates at one time if you design it like that.