Mumford & Sons, The Junction, Cambridge<br/>Grizzly Bear, Corn Exchange, Brighton

Despite the self-imposed handicap of a name that sounds like a firm of funeral directors – it is, coincidentally, the name of a fish and chip shop in Grays, Essex – indie folk quartet Mumford & Sons are unmistakably going places.

When you've been doing this job for long enough, you learn to sense when a band has plateaued and will never play larger venues than the one in which you're standing, and also to sense when they're strapped to the side of a rocket with a fizzing fuse. Mumford & Sons are palpably in the latter category.

On walking into The Junction, you're hit by a wall of sweaty steam that moistens your thighs and fills your lungs. All around are gaggles of neck-craning devotees who have learnt every word of the punishingly titled debut album Sigh No More, and glare at those who chit-chat through the quiet bits. This band, I quickly realise, is on the way to being Coldplay-sized, and will be skipping out of the Academies and into the Arenas in no time. When it happens, it will be with neither my blessing nor my curse. It's a dispassionate prediction. Put your money on Mumford & Sons for the 2011 Brits.

The question of realness inevitably rears its gnarly head when the F-word is involved, folk being a genre that prides itself on having directly traceable, aeons-old roots in the very soil of the land. For Mumford & Sons, nice south-west London public schoolboys in plaid shirts and bumfluff face-fuzz, this presents a potential problem.

It's difficult to sound righteously ethnic when you're of broker stock. But, truth is, t'was ever thus. The protagonists of the great folk revival of the late Sixties were already a crucial couple of generations removed from their source material. Fairport Convention weren't farmhands. Creedence Clearwater Revival weren't cowboys. Maybe there's never been a motherlode of authenticity where folk is concerned. Maybe it's always been an exercise in chasing elusive ghosts, and we should stop worrying and learn to love the ersatz.

In any case, there's plenty to enjoy. The Mumford sound, characterised by acoustic strumming sparkled up by glockenspiels and trumpets with the occasional eccentric "Solsbury Hill" time signature, has a rousing quality that recalls The Men They Couldn't Hang (essentially a poor man's Pogues). Marcus Mumford has a voice that reminds me, in turns, of Bright Eyes' Conor Oberst (on "Lend Me Your Eyes") and the Bunnymen's Ian McCulloch (on "Thistle and Weeds").

They're personable chaps, joshing about the way bassist Ted Dwane is only using a big word like "altercation" between songs because they're in Cambridge and suchlike, and I've a sneaking feeling that at least one of their tunes, "I Gave You All", could be one for the ages. One way or another, M&S shares are on the rise.

Don't be fooled by Grizzly Bear's relatively clean-shaven appearance: it's because they've done so much chin-stroking that their beards have fallen off. Like the Mumfords, the Grizzlies are well-to-do city boys whose music is informed by the folk tradition. Unlike M&S, however, the Brooklyn band's take on it is lateral, not literal, with even trickier time sigs, jazzy keyboard chords and plangent harmonies. It's all very 1969, in a Jethro Tull kind of way. It's doubtless difficult to play, and making their acclaimed third album Veckatimest was doubtless like knitting water. However, it's also somewhat dull.

Grizzly Bear are one of a succession of alt country-affiliated American acts – see also Bon Iver, Fleet Foxes and Animal Collective – whose popularity with British audiences has snowballed via word-of-mouth buzz in the past year or two. Taken individually, all these acts have their merits. Taken collectively, this whole aesthetic is beginning to stink like stale beer.

Easy as it is to despair of the gnat-like attention spans of the young, it's equally valid to despair at the arrogance of bands who behave as though spans are limitless, and refuse to recognise that attention must be earned. Grizzly Bear are guilty as hell.

Glancing around Sussex's self-consciously serious-minded razor-dodgers and their reluctantly-dragged girlfriends, I cannot for the life of me comprehend what anyone is getting out of being here other than to reaffirm their cultural identity as The Sort of Person Who's Into This Sort of Thing.

On 1,500 faces, not one smile. Everyone's pretending they're not bored out of their minds. The unquestionably talented but unfathomably tedious Grizzly Bear turn me into, to quote the Soggy Bottom Boys, a man of constant sorrow, and have me dreaming of a time when the last beardy, Pitchfork-approved Americana band has been garrotted with the last All Tomorrow's Parties lanyard.

Next Week:

Simon Price goes to Heaven, both literally and figuratively, to experience the comeback of MGMT