Please do not trot out the "Sondheim's music is unsingable/umhummable" crap argument that doesn't hold water at all. Everyone here can name dozens of songs that Sondheim wrote that fit that criteria.
Look, aside from some missteps (ahem, Cats) I DO admire Webber. I have recordings of every single one of his shows (including Cats even) and from time to time listen to them. However, the depth and intricacies that Sondheim puts into a score and lyrics is absolutely unparalleled in Webber shows.
Honestly, many of Webber's shows (Phantom, Cats, Sunset Boulevard, Aspects of Love, Starlight Express) are just the same six or seven melodies over and over and over (SB and Aspects being by far the worst offenders here). I fully admire JCS and Evita and Joseph, as well as his more recent shows, but that middle period is overblown and SO repetitive.
Let me put it this way - you listen to a Webber score once, you pretty much have it. I've listened to Sweeney Todd and Follies and Company etc. MANY times and I STILL find new stuff in them that I hadn't heard before.
In a way, the debate is almost stupid. Sondheim and Webber are as alike as Bruce Springsteen and Britney Spears. Both are mainstream, both are going to found in the same section of the music store (LITERALLY right next to each other), both have justly huge fan bases -- but when it comes to sheer artistry, there's very little competition. One speaks for a whole class of people, his music is influential and ranks up there with the greats of his genre, the other is pop candy - enjoyable, but ultimately without much substance (I guess by this analogy Hit Me Baby One More Time is JCS - that's gotta be sacrilegous in some way - but hey, I'm Jewish so happy Passover).
Hi, Shirley Temple Pudding.