htcdev does not allow writing of boot from recovery,so seperate kernel flashes are neccessary. you only have access to boot/system/recovery so you cannot change radios or hboots and you cannot backtrack to older firmware.
s-off allows access to ALL partitions,and allows the boot image to be written from recovery. you can change radios,hboots,and flash older ruus if you wish. it allows more options to recovery a soft bricked phone.
in short, s-off is better.
with it comes greater responsibility. make sure you double and triple check md5s of any file that contains a radio or hboot,as a bad flash of those can leave you with a permanently bricked device