If you ever want to be able to unlock your phone for use on another carrier then do not upgrade as it will upgrade your baseband to 5.11.07 which cannot be unlocked as of now and there is no time frame as to when and if this baseband will be unlockable.
If you anticipate you will not ever need to unlock your phone then upgrade.....although as the old saying goes "If it ain't broke dont fix it" so basically if you are happy with 3.0 and want to preserve your baseband for unlocking then stay with 3.0
3.0 is extremely stable and has no issues. Every jailbroken app works on 3.0. There is no benefit to anything higher at this point. Not worth your time and effort.