Autosomal DNA testing can be very useful for making contacts with lines that are difficult to trace, such as daughters of ancestors. The tests are very accurate up to 3rd cousins. However, more distance relationships may be discovered if you and/or the matching party have posted pedigrees.
23andMe offers a very comprehensive test for $99. Relative Finder is the database which matches people with relatives. The test also gives well-resolved mt- and Y-haplogroups, ethnicity and health testing.
FTDNA's Family Finder is similar, but does not report health results nor haplogroup assignments.
23andMe's Relative Finder database is now big enough to find close matches. I now have matches with 5 3rd cousins. vs 999 with 4th+ cousins. I have contacted one and received information on a female line that would been hard to trace, otherwise.
The best strategy is to test at 23andMe first ($99) and then upload to FTDNA ($89), unless you do not want the health testing. That way, you get into two databases for less than the cost of testing at FTDNA.