Looking museum ticket online, some mentions "skip the line" at the entrance - wondering if the ticket purchased on site, does it mean I need to wait in a long line? Trying to understand how much difference would the skip the line ticket make?
Also would be interested in seeing FC Barcelona game, is there any website would recommend for buying tickets?
"Skip-the-line ticket" is just marketing speak. Of course if you have a (legitimate) ticket in your hand, you don't have to stand in the ticket-purchase line. If you arrive without a ticket--risky for may sights in Barcelona--you will be in a ticket-purchase line, and it could be a very long one. And when you get to the head of the line you could discover they are currently selling tickets for 2 or 3 hours later (or conceivably for the next day).
These are the biggest problems in Barcelona: La Sagrada Familia, Parc Guell, Casa Mila/La Pedrera, Casa Batllo, Picasso Museum and Palau de la Musica Catalana (becaue the English tours at the Palau sometimes sell out). You could conceivably run into a bit of a ticket line at some other places (the large MNAC museum, the Miro Museum, Palau Guell), but I don't think they'd be all that long.
Unfortunately, Barcelona has a lot of sights for which you really need to pre-plan your visits. Unfortunately, an 11 AM entry ticket doesn't guarantee you entry at precisely 11 AM, so you need to be careful about scheduling multiple timed visits on the same day.
It's possible some of the highest-traffic sights (Casa Batllo and Casa Mila, perhaps), sell extra-cost tickets that can be used at any time and/or have some other benefit. I have no experience with such tickets.
