I have both the original Wilbour translation and the Norman Denny translation of this book, and I'd say that the Denny translation is the more readable of the two. Graham Robb, in his award-winning biography of Hugo has called Denny's translation "swiss cheese" and "translation as censorship." However, it's well-written, and the "excised" sections are included as appendices to which any reader can turn. In places where Denny edits the prose, he captures the spirit of the novel.
But the best comparison is made by reading:
here's Wilbour from the beginning of Part Two, Book Four:
"Forty years ago, the solitary pedestrian who ventured into the unknown region of La Salpetriere and went up along the Boulevard as far as the Barrier d'italie, reached certain points where it might be said that Paris had disappeared. It was no longer a solitude, for there were people passing; it was not the country for there were houses and streets. It was not a city, the streets had ruts in them, like highways, and grass grew along their borders; it was not a village, the houses were too lofty. What was it then? It was an inhabited place where there was nobody. It was a desert place where there was somebody. It was a boulevard of the great city, a street of Paris, wilder at night than a forest and gloomier by day than a graveyard. It was the old quarter of the horse-market."
Denny's version of the same passage
" A stroller forty years ago penetrating beyond the Salpetriere by way of the Boulevard de l'Hopital as far as the Barrierr d'italie, would have come to a region where Paris seemed to disappear. It was not a wilderness, for there were inhabitants; not country, for there were streets and houses; not a town, for the streets were rutted like country roads, and grass grew in them; nor was it a village, for the houses were too high. What was it then? It was an inhabited place where there was no one, a deserted place where there was someone, a city boulevard, a paris street, wilder by night than the forest, more melancholy by day than a graveyard. It was the anciet quarter of the horse-market, the Marche-Aux-Chevaux."