When choosing storage media, there are 3 things you are after - capacity, price, and performance. While capacity and price are easy to compare, it's never an easy task to compare the performance of a drive. Many factors affect the performance like chip type, OP, and controller, and the same drive may perform differently in different workload scenarios.
Here is a simplified meaning of some key performance measures
IOPS ( higher is faster) :
IOPS or input/output per second is the measure of how many read/write operations can be performed per second. Higher the IOPS, the faster the device. While HDDs have 150 to 180 IOPS, even the client SSDs can have a few thousand IOPS. Even if you have an array of 10 HDDs ( each with 180 IOPS), the array can reach a combined 1800 IOPS, much less than a typical SSD. FOB IOPS will be higher than the steady-state IOPS and they also depend on the block size.
Throughput/ Bandwidth/Transfer rate ( Higher is faster) :
The data that can transfer per second from the SSD. It's often measured in MB/Sec. The higher the Transfer rate, the faster the data can be read from and written to.FOB transfer rates will dip down to lower steady-state rates, and the rate is higher for bigger block size so there are these 2 parameters to consider also.
Latency ( Lower is faster ) :
The time that the host has to wait for to get a result of a command execution on storage media. The lower the Latency rate, the faster the storage media is.
Now the trouble starts when you start looking for an IOPS value, and realize that there are abbreviations before IOPS. Like FOB IOPS, Sustained IOPS, and steady-state IOPS.
FOB Rates are misleading, look for steady-state
An SSD performance best when it's FOB ( Fresh out of the box). This performance slowly degrades and reaches a point called steady state. Manufacturers understandably like to advertise the FOB rates ( which you will only experience for a few days depending on your usage) and slowly the SSD would settle to a much lower Steady-state.
Difference between FOB and Steady-state can be very high, so pay attention to the steady-state rates - that's what you will mostly have ( if you have optimum free space left on your drive)
Your Mileage May vary
Remember that SSD performance degrades over time as it fills up, and can vary greatly from one kind of workload to another. For example, sequential writing of large size blocks will be much faster than random writing of smaller size blocks. Test environments used by the manufacturers may be different than what your workload presents to the storage media, and the performance may vary accordingly.