It's hard for me to make a valid comparison here.
I just sailed the 2008 Isonic 111 yesterday for the first time, with a 6.5 m2 '08 Sailworks Retro and I seemed to be keeping up with everyone and passing most of them.
I have not sailed seriously since last summer so I'm sure my speed will improve as I get more accustomed to the Is 111.
The reason I cannot make a valid comparison is the Is 122 and 101 I had last year were both really good.
I really liked the '07 Is 122 and I think they've made it even better for '08.
Depending on what your conditions are (10-15 knots or 15-20 knots) most often, I'd go for the Is 122 if you get more 10-15 knot conditions, and the 101 if you get more than 15 knots more of the time.
More time on the water will allow you to tune the board and rig and sail your fastest.
So, let the "amount of time on the water in what conditions" be a major part of your decision process here.
Where did you run the mast foot on the Is 111? The '08 board I have seemed quite free yesterday, and I just put the mast foot in the circle on the board. I've always found the Isonics to be a bit more free and significantly faster (for me anyway) with the mast foot a little further back.
Hope this helps,