I prefer to use the suffix - it means that looking down the list of files in a directory is simpler: you don't have to mentally ignore the first four letters to get to something meaningful. (I'm assuming you have the tests in a different directory to the production code already.)
It also means that when you use Open Type (Ctrl-T) in Eclipse, you end up seeing both the production code and its test at the same time... which is also a reminder if you don't see a test class :)
