Actually the hardcore use both. Once things are put under ln2 they have problems posting at certain speeds/ its easier to change things on the fly since you only have a limited amount of time.
Software overclocking has a few pros to it, but for anything 24/7 i would never touch software stuff. There is room for stable overclocks on stock voltages (as far as i know, no program can change voltages on the fly), my i7 can hit 4ghz and be (fairly) stable on stock volts and i really dont have that much info on how stable software stuff is, although i can tell you that in no way will it be more stable than a bios oc.... and u dont get supah fast 16ghz windows start speeds.
Id love for some XS lurker to clear things up with the stability because i dont have that much experience with software stuff but here are my pros/cons:
Bios:
+Stable
+Faster Windows Start Time (maybe like .01 seconds but still)
+Voltage tweaks
+Some stuff u just cant change in software like CAS timing
+Easier to reset CMOS then go into safemode if u mess something up past the point of no return
Software:
+Faster
+Will always be able to at least post
+More user friendly
+???Just as reliable???
+Necessary for world record 1m super pi times
*edit*
Bios:
+Girls Will Be All Over You Cuz Its Cooler