What you are really looking for in a SDR depends on your needs. Here are some factors to consider:
ADC/DAC resolution: this is measured in bits. It pretty much represents how finely the analogue to digital converter or digital to analogue converter can represent changes in the waveform. Usually higher is better (12-16 bits), but you can get away with lower with some DSP techniques.
Sampling rate: this is usually measured in MSPS (mega samples per second) or KSPS (kilo samples per second). It is a measurement of how much data the SDR can convert from analogue to digital or digital every second. Again,higher is usually better,as it will be more versatile,and versatility is one of the most important aspects of a SDR (in my opinion).
Tuning range: this is usually reported as a range of frequencies. Pretty much self explanatory,it is how much of the of spectrum the SDR can tune to.
Duplex: this is usually reported as one of the following: half-duplex, full-duplex, and 2x2. A half duplex SDR can only either transmit or receive at the same time. A full duplex SDR on the other hand, can do both at the same time: transmitting and receiving. A 2x2 (or higher) SDR will be able to transmit on two different frequencies AND receive on two different frequencies at the same time. A 3x3 can do it on three,and so on. There is no "best" option here per say, but it really depends on your needs. If you don't need to transmit and receive at the same time, a half duplex is enough. However,if you do need to transmit and receive at the same time, such as a GSM base station, or something similar, you are going to need a full-duplex SDR. It really depends on your needs and your budget. A 2x2 is very versatile, but may be over budget and overkill in some situations.
Link type: usually reported as a name. The link type varies from SDR to SDR. The most common right now is Ethernet and USB. The advantage of Ethernet is that you can place the SDR in a farther away location, away from any EMI generated by your computer. You are unable to do that with USB, as the spec does not allow you to stretch it for a long distance. However, I find USB easier to work with.
Price: usually reported as a multiple of a unit of currency. The price shouldn't be the main deciding factor when choosing a SDR. You should look at your current needs first, while also considering any future needs. Then, you should consider price. It is much better to spend a little more right now on a beefier SDR than to have to buy a new one down the road because it doesn't have the specs you need for a particular project.