I agree the two methods of searching seem to work differently, but I can't agree the "similar threads" is a better search engine.
First, I am not sure what is meant by this:
When you are using search you need write exactly phrase.
As a test, I used Advanced Search to search for these keywords in this order: "searching, post, edaboard." Clearly, those exact words do not appear in that order in either the title or body of posts here, but Advanced Search found this thread.
I then entered those words as a title and got the similar posts listing shown here:
The similar posts search engine did not find this thread.
I suspect the similar posts engine only looks at titles as gulson said; whereas, the advanced search engine can look at titles or whole body. I consider the latter an advantage over the former method. Both methods are keyword based in the usual sense and neither seems to be dependent on the order of the words. This little experiment didn't test whether either engine finds "similar" words versus exact words or word parts (e.g., "search" would return both "search" and "searching", but "searching" would not return "search").
While on this subject, it would be nice to be able to do a second search within the original returned items from a first search. In other words, be able to add a filter to the returned hits, much like the parametric search engines allow on the DigiKey and other vendors' sites.
John