The 2012 is only 1mm thinner than the 2012 one (19mm to 18mm) they are both the thin version of the MBP all the retina ones are. The difference in weight is also marginal. They are both huge step up in thinnes and lightness from the classic MBP.
However the main reason for getting a 2013 is the huge boost in graphics that intels IRIS gives over the HD 4000 in the 2012. Plus the Thunderbolt 2 and AC wireless card are a bonus.
If you can get the 2012 at a really good price though then why not.
That extra 1mm made it so chunky, they made sure they only produced that fat beast for one year before making it thinner.
Intel IRIS graphics is the big bonus.
Plus the PCIe SSD can be quite a bit faster than the previous SATA interface SSDs.
Just one word of warning - if you get the late 2012 rMBP there are a number of options to upgrade the SSD with third party products available from Transcend and OWC, plus some novel solutions such as this one using an mSATA adapter:
https://forums.macrumors.com/threads/1836345/
With the late 2013 - mid 2014 model rMBP, you really need to make sure you purchase it with the SSD you want as there are no third party SSD options. At best you might be able to get a Apple O.E.M. SSD made by Samsung on eBay.