I'd recommend making a list of places you will definitely visit and places you hope to visit, then comparing that list to what the Barcelona Card covers. The last time I checked, it didn't look like it would benefit most visitors. Some of the most popular (and expen$ive) sights only had slight discounts with the card. Here's the list. Note that Casa Batllo and Casa Mila just have 3-euro discounts (hardly worth mentioning), Sant Pau is 20% and Palau Guell is 25%. A lot of sights now offer a slight discount when you buy a ticket online, so the additional benefit of the card is probably no more than a very few euros for a lot of the most popular places. All pre-purchased, timed tickets give you skip-the-line access, the same as the Barcelona Card. The vast majority of sights in Barcelona are easy-access and do not require you to buy a ticket in advance.
It's easy to buy a T-Casual ticket to use on buses and the Metro. It's good for 10 rides and costs only €11.35. Barcelona is a very attractive and walkable city. I don't think most visitors take that many rides on public transportation, so to me the transportation component of the Barcelona Card isn't worth all that much.
The Articket BCN looks like a good deal for folks who expect to spend a lot of time in Barcelona's art musuems. It lasts for a year and allows multiple visits, something that might be advantageous at the fairly large MNAC. Just be sure you'll have time to visit enough of the museums to make the Articket pay off. Are you planning to spend seven days in Barcelona, or will you be taking day-trips to Montserrat? Girona? Figueres (Dali Theatre-Museum)? How much time will you have left after hitting the modernista sites?