Keeping the SDR cool seems to be the most important thing. So a metal cased unit where the tuner IC is thermally coupled to the case is what you'd want. Then it's on you to keep the case cool, i.e. suck the heat off it somehow. That'll keep the tuner cool and improve reception.I've noticed on Amazon different SDR's One has the R820T tuner, the other R820T2 tuner.
Is there any real difference? The only thing I'll be using it for is ADSB.