HN and Ars seem to be complementary, HN gets links to things that Ars wouldn't report on and Ars reports on things that HN wouldn't get links to. There's overlap but that doesn't detract from going from one site to the other.
HN makes Slashdot somewhat redundant. When I read Slashdot, a good chunk of the articles are links to things that appeared on HN three days ago.
Also the Slashdot editors can't edit for shit. I guess they're too busy posting thinly-veiled advertisements for Dice.
If the commentary on Slashdot ever became less informative (although the signal to "Micro$oft $hill!!!!" ratio is decreasing...), then I'd leave.