Are ther any benefits to having 120 real stealing skill vs. say 100 stealing increasing to 120 with artis?
Well if you want to know if you're better off with real skill, it depends what sort of kit you plan to use and whether you insure it.
I personally work in newbie clothing most of the time, so I avoid items like the plague. If you're planning to venture to Fel for some player robberies, you may want to use real skill to reduce any insurance payouts to your marks. If players suss that you can't give counts and pay out insurance, I find the death count goes up quite significantly
As for the general benefit from having 120 stealing, someone can correct me if I'm wrong, but I'm sure 120 guarantees that you'll successfully steal a doom artie etc. I ran with 115 for ages and failed at times, but at 120 I was told you never fail. For a desirable artie *cough* books *cough* then 120 could be useful where competing thieves are all after the same item. I've upgraded to 120 but haven't done the training yet to be certain.
You will also be able to pull the new monster stealable items more regularly too.
Your stealing skill also enables you to steal increasingly heavier weighted items. So again 120 would give you an advantage.
Hope that helps
Wenchy