speaking of a hard drive, what is more important? RPM or the read speed?

You have misunderstood one of those specs. The 600 and 300 numbers you cite are NOT Read Times. They are versions of a SATA spec.

There have been three SATA versions so far, distinguished mainly by the MAXIMUM data transfer rate of the communications channel between the HDD's control board and the computer's HDD controller chip. The original SATA spec was 1.5 Gb/s (that's 1.5 Gigabits per second, and notice the BITS part). Its successor was known as SATA II, now renamed SATA 3.0 Gb/s. The most recent is SATA 6.0 Gb/s. Now in all three of those speed specs, some people approximate that one BYTE of data consists ROUGHLY of 10 BITS, so 3.0 Gb/s can be "translated" to 300 MB/s - note the capital B for "byte" - and the most recent spec looks like 600 MB/s.

Now, the reality is that on any mechanical HDD - one with rotating disks and moving heads on arms - the mechanical movement is MUCH slower than the maximum speed of transfer of data along the external communication channel. This is by design - you never want the comm channel to slow down the overall performance. In actual measured performance over a wide variety of tasks, today's mechanical HDD's can move data on a sustained basis at about 130 to 200 MB/s, and a FEW might exceed that. So NO mechanical HDD can actually use all of the max data transfer rate of the SATA 3.0 Gb/s version. The only devices that can beat that limit are the newest SSD's - Solid State "drives" with no moving parts to slow down - that can get over 3.0 Gb/s, but not yet close to 6.0. That is one major reason why the SATA 6.0 Gb/s newest spec was developed - to allow very fast alternatives to mechanical HDD's to operate at their best speed, unhindered by the max data transfer rate of the communications channel.

So, when it comes to rotating-disk mechanical HDD's, it really does not matter whether the unit claims to be SATA 3.0 Gb/s or SATA 6.0 Gb/s. The unit cannot achieve either of those speeds in actual data transfer tasks. IF you go with the new SSD's, then you really do need to go with the SATA 6.0 Gb/s system. All such SSD's are that way, but ideally you would need to connect them to a mobo SATA controller system that also is rated as SATA 6.0 Gb/s.

Now, rotational speed IS important on a mechanical HDD. It determines how long it takes for the first Sector of a file to rotate around to the stationary head position so the unit can start reading the file. That is part of "Head Seek Time". It also determines how fast the many Sectors of one track can pass under the head, and that is a big part of the data transfer rate. So a 7200 rpm HDD is faster at data transfer than a 5400 rpm unit.

Another factor in performance is the size of the HDD's on-board cache memory. That is important for many HDD tasks, especially random reads and writes, although a little less important for long sequential tasks. Like many things, more is better up to a point. Many find that, for HDD's of the same rotational speed, a cache size of 32 MB is noticeably better than 16 MB, and 64 MB is faster yet, but not by a huge amount. A design trick that is used (because it works!) on the 5400 rpm (slower) HDD's is to provide it with the larger 64 MB cache to help offset the speed disadvantage of the slower rpm. This combination still does not make it as fast as a 7200 rpm unit, but it helps.