I'm looking to buy an SSD for a SQL Server computer, what benchmarks should I be comparing? High/low queue depth? Random/sequential transfer rates? IOPS?
IOPS pretty much.
Queue depth is irrelevant - basiaclly some higher end systems work better with more stuff queued. This wont give you any sensible information without context.
Transfer rates depend on IOPS - someone has to read / write the stuff first.
At the end, random IO, always runs down to IOPS. IOPS directy transfer to transfer rates when needed. Note that SQL Server ALWAYS does 64k IOs or multiple of that for enterprise. So, 4kb etc. is not relevant. From that you can pull all other information. Response time too slow, queues up - not enough IOPS available.
The benchmarks you want are IO latency for the following workloads:
You might also be interested in the throughput (MB/s) of these workloads:
For all the writes, bear in mind that caching to volatile memory before IO hits the disk can make MB/s seem higher than it really is. SSDs are as good at random IO as they are at sequential IO.
The short answer is "It Depends"
Unfortunately without knowing your usage patterns it is almost impossible to suggest an "ideal" solution.
Some things to keep in mind