They're all basically the same with respect to two aspects (because these are the two aspects that matter the most):
1) The most popular "model" is made with the R820T tuner by Rafael Microelectronics so as long as it's using that tuner. You'll see R820T quite often with respect to SDR hardware for the low cost side of things.
2) The most popular controller chip - for USB communications to and from the device itself - uses the Realtek RTL2832U chipset hence the extremely common use of "RTL" when referring to these types of devices.
If any stick you're interested in looks like this you're pretty much ahead of the game already because these are arguably the most popular ones that fulfill both aspects above:
There are several other variations out there too:
- the Elonics E4000 tuner version, which has a wider frequency response overall - I think it's like 60 MHz to 2.4 GHz - but Elonics went out of business so they're no longer made and considered quite rare nowadays (the Teratec sticks use that type of tuner chip, at least some of the models) and their pricing reflects the scarcity: they usually sell for $50+ and that's if you're fortunate enough to find a reseller that has some (even one) in stock and you're willing to fork over the cost
- the FC series of tuners (not even sure who makes them) is the other mildly popular tuner that's not nearly as well supported for whatever reasons, and there's several versions
You can find a lot more info on this page from the RTL-SDR wiki (which if you note refers to the Realtek chipset again, the sheer overwhelming popularity of those particular chipset based models puts it high enough that the entire community just refers to itself as RTL-SDR nowadays):
compatibility - RTLSDR
tl;dr version: Get one based on the RTL2838U with the Rafael R820T tuner, which won't be tough to do because - again - they're so popular. NooElec is probably the most popular reseller in the US as well - I got 2 RTL sticks from them recently through their Amazon storefront for $9.75 each although the price does fluctuate and typically is about $17. I don't know why I got mine so cheap but in December that's what the price was so I bought one, got it, then ordered another one 2 weeks later for the same price. I got lucky in that respect and for a roughly ~$23 investment I can now actually monitor 12 channels at the same time using both sticks if I really wanted to go that far (courtesy of info I just got from another thread - THANKS SCOTT!!!
)
Biggest piece of advice: get two and get pigtails so you can attach real antennas when you get them - the tiny little 5" thing they call an antenna that's included is almost a disgrace. Sure, it'll allow you to get up and running, test the hardware and tune in a few things in the upper frequencies (UHF and higher) but it's nothing to bother with if you find yourself seriously interested in this new way of doing things.
And of course it goes without saying that you
need good antennas, whichever one(s) you find most useful whether they're commercial/retail models or coathanger 1/4 wave ground planes you make yourself which is what I did and it works fantastic. RTL stick (or sticks, as noted), pigtail(s), and good antennas coupled with the great software we've got available and it's a win-win-win.
Good luck...