in a nutshell: s-off=security off,and unlocks all partitions of your phone,not just the boot,system,and recovery that htcdev unlocks.
it means you can change splash screens,bootloaders,and radios at will,as well as flashing older official htc software. its also considerably easier to flash roms from a permanently installed recovery without a PC.
the downside is,you must use extra caution when flashing,as flashing a bad radio or bootloader WILL brick your phone beyond fixing.
htcdev,while it has limitations,does allow alot of user error,as the bootloader and radio are protected from accidental,or intentional flashing. its more or less imposible to "brick" a S-ON rezound with beyond repair,with software flashing since only boot,system,and recovery are accssible via htcdev unlocked
s on hboot