Photo by Xavi Torrent

Barcelona's Cruilla is a music festival in the same way that your aunt once bought you a Guns N' Roses CD because she heard you were into music. That's to say it takes music as one indivisible entity, with no concern for underlying themes or niche trends. If you wanted a British comparison, Cruilla is a little like Wireless meets the Pyramid Stage at Glastonbury: a mixture of rap and R&B, pop acts, a sprinkling of indie, reggae legends and what, for want of a better word, we might call "world" music.

This year, though, there is a kind of logic to the twin headliners, Kendrick Lamar and Lauryn Hill: both acts have gone from a background in simple, commercially-appealing rap into making music that is darker, weirder and more politically aware, taking cues from jazz, gospel, soul and funk. 

The big difference is that Kendrick is still high on the wave of this year's brilliant To Pimp A Butterfly album, while Lauryn Hill has been in a whirlwind of exile, tax evasion and disappointing acoustic albums since the release of her groundbreaking debut album The Miseducation Of Lauryn Hill in 1998. In summer 2015 Kendrick Lamar can seemingly do no wrong, while Lauryn Hill's last London dates saw the singer slated for errant time keeping and a lousy sound mix.

Kendrick headlines the Friday night (right after Jamie Cullum, for some reason) and proves a vague disappointment. To Pimp A Butterfly might have been a leap forward in musical experimentation and ambition, but Lamar's live show has yet to catch up. He still uses essentially the same live set up as the last time he was in Barcelona, in 2014 - a small band plus backing track - and the setlist is dominated by songs from his breakthrough album, 2012's Good Kid, M.A.A.D City.

It seems slightly churlish to complain: the sound is crystal clear - a rarity for big hip hop shows - Lamar's rapping is sharp and lyrical and the band are taut and funky. The sizeable crowd loves it too, screaming along in marked Catalan accents to big hits like 'Swimming Pools (Drank)' and 'Bitch, Don't Kill My Vibe'.

And yet, when compared to the freewheeling musical odyssey of To Pimp A Butterfly, an album that drinks in jazz, p-funk and psychedelia and cries out to be explored live, Kendrick's current live show feels a bit too comfortable, with the rapper operating in a well-worn groove that To Pimp A Butterfly suggested he had transcended. It's like Parliament in 1974 going back to doo-wop: all well and good but you know they can do weirder. 'King Kunta', one of the few Butterfly tracks that does get an airing, is brilliant, an injection of colour and off-beat energy in an otherwise rather one-paced set.

Lauryn Hill, who headlines Saturday, is anything but comfortable. In many ways she offers the polar opposite of Kendrick's slick, hit-filled set. For a start, she kicks off 35 minutes late to a chorus of boos after sending an unfortunate DJ to warm up for her with a selection of reggae, hip hop and unspeakable EDM. The sound, too, is muddy compared to Kendrick's crystalline mix and Hill spends the first few songs glowering at the sound man and making "turn it up" gestures, as her backing singers continue inaudible.

Hill doesn't exactly lean on the obvious hits, either (at least in the first part of the set). When she finally arrives on stage, she takes a seat, pulls on an acoustic guitar and launches into the unreleased 'Conformed To Love'. It is five songs before she dips into The Miseducation… and then it is for a reggae take on 'Ex Factor', which bares so little resemblance to the original that you find yourself scouring the lyrics for evidence that this is, in fact, the much-loved song from her 1998 debut.

Her set has all the ingredients for a disaster. And yet, as Hill tears into the songs with a reckless intensity, as if furious at a delay you imagine was largely her own fault, you realise why people are still prepared to take a chance on the former Fugee. Not only is she's a singer of incredible talent but she has that rare gift of making you share her feelings. It may all be illusion but you feel like Hill simply has to be up there on stage, on a  balmy July night in Barcelona, or she might explode with emotion. It is an intensity of feeling that it can't help but make all other acts look a little phoned in.

It helps, of course, that she starts to roll out some better known songs as her set advances - 'Fu-Gee-La', 'Killing Me Softly', 'Ready Or Not' and a breakneck stomp though 'Doo Wop (That Thing)' among them. But it's really not so simple as hit songs = better gig. Even during the earlier, unreleased songs, with Hill pretty much ignoring the audience in favour of haranguing the sound man, her raw intensity was evidence of a special talent, albeit one that needs indulging.

That I spend the next 24 hours dreaming of a Kendrick Lamar / Lauryn Hill supergroup - one with the emotional heft of Lauryn Hill and Kendrick's sharp, crowd-pleasing nous - is a sign of both acts' obvious strengths as well as an inevitable tribute to the power of FFS, who follow Lauryn Hill on Cruilla's main stage.

FFS, the inspired combination of Franz Ferdinand and Sparks, put the fab in collaboration and the super in group. In a festival setting the group's genius is that each half of the pairing provides exactly what the other lacks: Franz Ferdinand give Sparks some festival-friendly live oomph, courtesy of Paul Thomson's thundering drums and the rhythmic assault of Nick McCarthy's guitar, while Sparks give Franz Ferdinand - a sometimes great band, who seem to have lost their way - a route back into their art pop roots. Dual vocalists Alex Kapranos and Russell Mael spur each other on to greater heights of exhibitionism and showmanship, which might look a little overplayed in a basement in Glasgow but are exactly what is need at 2am on a massive festival stage when everyone has been drinking since noon.

FFS, then, are a brilliantly ostentatious festival band, one that embraces the ridiculousness of rock music without losing sight of its tempestuous power. The biggest cheers, it is true, are for the band's individual hits ('Take Me Out' and 'Do You Want To' versus 'The Number One Song In Heaven' and 'This Town Ain't Big Enough For Both Of Us') but FFS efforts like 'Piss Off' and 'Collaborations Don't Work' aren't far behind.  The biggest cheer of all, however, is for Ron Mael's three dance moves, which he breaks out at the front of the stage for 'The Number One Song In Heaven' after a gig spent behind the keyboards in his typically statuesque stance. 

It is a ridiculous, joyful highlight of a strange night at a peculiar festival, where Jamie Cullum shares a bill with Kendrick Lamar and Lauryn Hill cedes stage to FFS. Sometimes, though, as Cruilla proves, weird can work. Take note, Kendrick.