Hi Doug,
I can't comment with any authority on STI, but I do have a Club Tour with barends and 3 bikes with Campag Ergolevers.
Some random comments:
Barends are easier to use when on the drops (I have small hands).
Ergos are obviously easier to use on the hoods .
Barends can trim the front derailleur very effectively. This can be an issue with Ergos and a triple chain ring. STI's may be better at this.
Barends can shift across several cogs in either direction in one movement. Older Ergos do this but not later versions. I don't know if STI's do this or not.
The position of the bar end levers is a good indicator of which gear you are in. This can be useful towards the end of a 200 KM Audax ride in hilly terrain.
My Club Tour has travelled about 30,000 kms (touring, Audax, commuting) with no barend issues and they are virtually maintenance free.
Some people don't like barends because they can hit your knee when you're out of the saddle. Let me assure you it only happens once!
I hope this helps....
Enjoy your Mk3
pj