If you can afford to wait, do so. Rumors (and many threads) suggest that IF a 13" rMBP exists, it will be released between Oct-Nov (pre-Holiday Season) or during the next Intel refresh (Haswell update).
If you don't need/want a Retina display, or any other specification that is rumored to come along with the upgrade (discrete graphics or even quad core), then the existing 13" is a great option.