On the day that her second album, Drastic Fantastic, went on sale, KT Tunstall played a semi-secret pretour warm-up gig at Wilton’s, in east London. Though she handled the occasion with her customary chatty aplomb, and turned in a thoroughly enjoyable performance, there was a palpable sense throughout that Tunstall’s career is in a transitional phase, and that some tough decisions have still to be made.

The first being, should she present herself as the high-kicking, glammed-up, silver-guitar-slinging rock chick portrayed on the sleeve of Drastic Fantastic? Or should she hang in there as the scruffier indie troubadour who came to the public’s attention playing a solo set on BBC2’s Later . . . with an acoustic guitar and a loop pedal?

Tunstall is hardly the first female singer-songwriter to have faced this conundrum – Michelle Shocked and Sheryl Crow both grappled with it in the 1990s. But the scale of the success of the 32-year-old Scot’s debut album, Eye to the Telescope, which has sold more than 4m copies worldwide, makes the need to resolve it pressing.

Most of Tunstall’s fans live in America, which is presumably why she played the US leg of Live Earth rather than Wembley. The gold leggings she wore at the Giants stadium seemed designed for a transatlantic audience that doesn’t really go for dressed-down tomboys with a wry, self-deprecating sense of humour. This same audience prefers the full-bodied big-guitar sound of Drastic Fantastic to the lighter, more rhythmically nimble and stylistically eclectic range of Eye to the Telescope.

The KT Tunstall who turned up at Wilton’s with a six-piece backing band was diligently trying to square the circle. She came on in blue jeans, waistcoat and scarf, looking and talking like Kirsty Wark’s younger, groovier sister. She immediately launched into the opening track from her new album, Little Favours, a song that Fleetwood Mac inexplicably never got around to writing. “If you thought that was shite, we’ll play you an old one,” Tunstall advised in the first of several witty asides, the best of which neatly summed up the experience of using a drum machine as “like having your dad on stage with you”.

This pungent mixture of sarky Scottish backchat and bright American-freeway anthemics came together nicely on Saving My Face, a new song about plastic surgery that Tunstall suggested might have been titled Pimp My Granny. Elsewhere, though, notably on the country-rock chugger Hopeless, the singer and the song felt a bit too comfortable in their borrowed cowboy clothes.

If Tunstall wants to follow a country road, she certainly has the vocal power, songwriting smarts and sassy personality needed. But it would be a terrible shame if there were no more songs of the soaring melodic quality and elusive style of Suddenly I See and Black Horse and the Cherry Tree, two of her older numbers, which brought the 45-minute show to a close. There was a touching gaucheness in the way she threw her plectrum into the audience at the end, which implied that Tunstall still isn’t quite sure what kind of rock star she wants to be. Whichever way she decides to jump, she has more than enough talent to pull it off.

Editor's choice

• The 100 Best Films of the Decade
• 70 facts you didn't know about Marvel
• The 50 Biggest Movies Of 2010