To flash any of these, first establish an activesync connection via usb. Then run diamond-sspl.bat and confirm execution on touchscreen. The screen will turn black. Now run DiamondCustomRUU from either the HardSPL-1.23, HardSPL-1.24 or HardSPL-1.34 folder.
I have tested these versions on my diamond and did not have any issues with them. I prefer 1.24 because it is a mfg spl. A mfg spl includes additional commands for the manufacturer (htc) to test a device as opposed to the shipped spl which is installed on stock devices.
A particular spl may fix bugs, may be needed to support new hardware (e.g. lcd or touchscreen) or is required to optimally support a particular version of the OS and/or radio. Only try another spl if you have issues with your currently installed one. If you have no issues then don't flash another spl.
Further questions about spl are answered here
1. cmonex for the wonderful relocation hack in sspl
2. pof for JumpSPL