Over on the hamradioscience blog the author has uploaded a post showing a comparison between the R820T and the R820T2 tuners.
Recently Nooelec began selling RTL-SDR dongles with the R820T2 tuner chip in them (Ebay). The R820T2 tuner has been anecdotally reported as having slightly better sensitivity (~2-6 dB better). It also works much better with Oliver Jowetts experimental RTL-SDR driver which enables HF reception without any hardware modifications.
In the post the author compares the two tuners by simultaneously running both with SDR# and the same antenna type and comparing the results. From his results the author writes that although not night and day, there is a noticeable difference in sensitivity between the two tuners. He also writes that for ADS-B the R820T2 performed much better, reporting significantly more packets and further distances compared to the R820T.