They are good composers, no doubt, but probably not quite in the same league in, if anything, importance (not going to argue personal preferences in music) as, say, Beethoven earlier or Stravinsky later.
Not to say that say Mussorgsky (as mentioned somewhere above) isn't a top-notch composer, but if you have, as your comparatives, Dostoyevsky vs. Dickens (for example) or Mussorgsky vs. Wagner, you'd probably pick Dostoyevsky...