Each of these engines have their own characteristics and strengths. Try searching several engines for something very obscure and specific. Then take a look at the first 20 or so entries that each engine returns. Are they on point? Do some engines give enormous number of hits while other relatively few? The ones with relatively few hits are probably using artificial intelligence techniques and may be better for your purposes.
We at Armchair can't understand the value of a search engine reporting 10,000 hits while giving no means of examination except sequentially viewing the results 10 or 20 at a time. Expect this to change (for the better) in the near future. Maybe by the milleneum....
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