I've picked Juve, but I think it's important to understand Barca. (I'm excited about this matchup)
They are not the "tika taka" team of past champions. They are not reliant on finding Messi any longer either. If you haven't watched Barca this year, find some you tube videos. Luis Enrique has done a marvelous job with them, and took a massive gamble in abandoning Pep/Cruyff's dictum on using possession to defend.
They are set up in 3 very distinct zones:
Defensive area... behind half line. Unlike the 2011 championship team, this Barca absorb pressure inside of their own half. They no longer high press (win the ball in 7 seconds) as Pep had them doing a few years ago. They will be as likely as Juve to have 7-9 behind the ball at all times.
Midfield zone: (actually three areas) Barca's midfield is no longer the evolving triangles linking the 8 lines of Pep (and Cruyff's) 433. (where defenders, mids, attackers created intermingling triangles all over the field). It is now a solid rotating triangle of three midfield players that spin like a chinese star across the field. It's initial purpose is to disrupt opponent possession. Its secondary purpose is to push the ball forward. its third purpose is to shadow the back line. its fourth, to add to the attack. It no longer jumps into the other teams attacking zones to chip away and unbalance teams as it did in the past. Therefore, it is far less likely to get countered on (as it did in the past). Macherano/Busquets, Rakitic and Iniesta typically comprise the rotating defensive buzz saw. It is very important to note their responsibilities in order... 4th is join the attack. Very rare for a Barca team.
The attack: Neymar, Suarez, Mess... christ on a bike...
They are alone, a zone amongst themselves and pressure the opponents almost exclusively as a 3 man unit. They do not high press, so conserve their energy for the attack, and only check back to the opposite side of the ball. They are constantly poised to spring to life, like a boulder always on the tip of the cliff. Teams cannot press forward on them or they will be left exposed. Right, left, center, somewhere there is space for one of the best attackers on the planet to run at you (take your pick). This is where Lalas is wrong... (shocker). These three typically operate as lone gunners in their own channels, running around and off each other. The midfield doesn't need to get involved more than supplying them with 20-25 yard passes. There is no false 9 to start the link up play. Lalas still thinks this is 2011 Barca.
Juve's challenges are substantial here. While they may be able to get numbers behind the ball, Barcelona are just as apt to move the ball to their attacking three, anywhere along the line, and just let them go. This is where the chess match, and key battle takes place. Juve's 5 man midfield (and subsequent 3 man center) will need to chop up Barca's and ensure the ball does not get to their front three. They were able to do this against Real to good effect. But Real is a different beast, and not as effective in matches where varying philosophies may take effect. Juve will need to be patient as well. The more they move players forward, the more space for balls to Neymar, Messi and Suarez will be found. And unlike previous Barca squads, the Spanish champs can play balls over the top to the waiting Suarez at any time, something they've never been able to do in the past. Juve's 3 man back line may be fodder for a player like Suarez alone. Add Neymar and Messi, and its a tricky game of chance for Juve.
Here is why I think Juve will (could) win..
Their front line will get chances against Barca's group, as they did against Real. If they make them count, or even if they scare Barca enough to start dropping Messi and Neymar away from goal to help clog the wide spaces, it will give Juve time to drop and defend in bunches. Juve's wingbacks will fly up and down those channels, and either Barca will allow it, or ask 2 of the best attackers in the world to defend (something Ronaldo and Bale would not do).
I think Juve's 3 man group of Pirlo (the beard), Pogba and Vidal will overpower Barca's center, over time. They won't take chances unless they are behind, and even then, it won't be until late (they are Italian). The more possessions they win, the more Asamoah and Marchisio will be able to push forward. The more the wings get up, the more Barca must decide what to do with Neymar and Messi (hold them high, or drop them back). If Barca lose the center of the field, and Juve can exploit the gaps between the wingers and wing backs, the italian side can win.
It will be a tactical game, a chess match, 433 vs 352, and I'm excited for it.
