Two big stars; two kinds of music; and, as if to underline the stylistic differences that still divide that music, two conductors. If Bryn Terfel and Renée Fleming can sing Wagner, Strauss and Mozart alongside Gershwin, Rodgers and Sondheim, why, you might ask, couldn't either Gareth Jones or Paul Gemignani – conduct-ing the Orchestra of Welsh National Opera – do likewise? And here was I thinking that the word "crossover" had gone out of fashion.
One key issue divided Terfel and Fleming in the operatic extracts: words. A sneak preview of Terfel's forthcoming Wotan in Wagner's Ring – the "Farewell" scene from Die Walküre – found the big bass-baritone already making that crucial distinction between god-like authority and fatherly concern. Few singers today are so completely "contained" on stage or concert platform. There is no hint of affectation in Terfel's work, just an instinctive honesty. He knows exactly what he is singing about, and so do you. And even where the showman and natural comic surfaces – as in Leporello's "Catalogue" aria from Don Giovanni (only Terfel would have thought to use the concert programme as a prop here) – it's always about making sense of the text.
Fleming, on the other hand, has become more and more about la voce. In the closing scene of Strauss's Capriccio, the Countess ponders the age-old question: is it the words or is it the music that moves her heart? With Fleming, you are never in any doubt. The words are always subservient to the sound, vague and featureless where they should be tantalising and ironical. How strange to have the dynamic of this most celebrated scene removed before even a note is sung. The Fleming sound is all about "spinning". Her seamless, all-purpose vocalise slides seductively from one note to the next like a piping of extra cream on to an already rich confection.
This sliding – or portamento – is a very mixed blessing indeed. In "Porgi amor" from Mozart's The Marriage of Figaro, where the beauty is in the purity and brevity of the aria, Fleming's crooning spoke only of immodesty.
But then something interesting happened in the "show tunes" part of the programme. Fleming became a different singer. True, these were native songs in her native tongue; also true, with the help of a little amplification, the chest part of her voice, the "speaking" part, now came into play. But her singing at last began to say something. "Loving You" from Sondheim's Passion may have leaned a little closer than is comfortable to an operatic delivery, but the simple gravity of the song really made an impression. Then there was her Bess which, dare I say it, upstaged even Terfel's Porgy for the bluesy audacity of the singing. Now she could croon to her heart's content.
Not everything in this half of the concert quite made that stylistic leap of faith I spoke of earlier. With the best will in the world, even showman Bryn couldn't quite convince me that he has left his operatic credentials at home as he "oompahed up and down the square" to the strains of "Seventy-Six Trombones" (a rowdy arrangement by Rob Mathes, who managed to wreck "Hello Young Lovers" from The King and I by turning it from an old-fashioned waltz song into a smoochy torch song). But his booming rendition of "Stars", from Boublil and Schonberg's Les Misérables – which can use all the voice you can give it – rightly brought the house down. And if he and Fleming can still convince me that the gospel soul-searching of "Wheels of a Dream" from Flaherty and Ahrens' Ragtime is no more of a stretch than Gershwin is from Wagner, then I'm all ears.
To be re-broadcast on Radio 3 in December