If you're sure you only going to use them as overheads using AB spaced pair them you can decide to buy unmatched. However if you want an X/Y placement for your overheads or if you want to experiment with stereo miking in general you're going to need a matched pair to reduce phase differences.
I've got the 603 S matched pair and I like them very much. I use them alot on acoustic guitar. I'm about to use them as overheads in a few weeks. They s*ck on vocals! Especially mine