The extreme love that IWW gets is almost as revisionist as the RD dikkriding.
I love IWW, but it was heavily criticized for not living up to the illmatic standard, for being commercialized, for following the mafioso/kingpin trend, for the poppy samples, for having radio friendly songs, for being less "pure hip hop," for having less premier and co. and more Trackmasters, etc.
illmatic is without a doubt better. IWW is still dope.
Best way I can describe it, and the best analogy I can think of is:
illmatic = GoodFellas...groundbreaking, raw, organic, ahead of it's time, start to finish it's hitting on all cylinders
It Was Written = Casino...a more polished version of it's predecessor, more glamor, it has some lag/down time, fells less organic than the predecessor, etc.
There are ppl that like Casino/IWW more than GoodFellas/illmatic...they're not wrong or anything, but the majority prefers that raw, organic, unadulterated, uncut product. Casino is classic, but it took some time for it to reach classic status, critics and fans felt that it was missing a certain "it" factor that GoodFellas had.