The journey from the 5 Act French Grand Opera performed in Paris to the 5-Act Italian Modena version was quite a long one for Verdi. The only constant was the French language, something that tends to be forgotten nowadays. He always regarded the Italian version as a translation and that the French libretto had primacy. But the 5-Act Italian version of the opera was premiered nearly 20 years after the opera's original premiere in Paris (in 1867).
So the later version of the opera is a finer, tauter thing; more dramatic, less leisurely, closer to later Verdi. The earlier opera is discursive (it was too long even for Paris and was cut prior to its premiere there), and Verdi's ultimate take on the genre of French grand opera. That particular art form which was developed specifically for the Opera in Paris.
For those interested in the opera's history then, it would make a lot of sense to try performing the original version of the opera (even in a cut version). This was done by the BBC in a concert performance in 1976. This is going to be re-issued by Opera Rara in their Verdi Originals series and I can't wait for it to come.
But most opera companies and comentators cannot bear to perform, in unrevised form, passages which were subsequently revised and tautened up by Verdi; the master's final thoughts are paramount. There is, of course, no problem with this; it is as it should be. But there is a lingering fascination with the bits Verdi cut when creating his later versions of the opera. So WNO, like the Royal Opera before them, perform the Modena version with bits of the original Paris version inserted. The result makes a mockery of Verdi's cutting and tautening and causes tension between the two eras of the opera's performance.
Any performance of Don Carlos is an event and one in French is very special, no matter what the edition. But I wish that opera houses would stick to a version of the opera which bears some resemblance to one that Verdi intended.