Borderline case !!
Typically with 80kg rider, the iS101 is going to be fine with 7.3m, and the upside of the 101 choice being that you will certainly get further up the wind range (and thus a noticeable increase in quiver range) with 101, while still retaining minimal but practuical overlap to the 133, UNLESS you're often in marginal 7.3 conditions (in which case the 111 is king).
Typically quiver sizing in iS's goes in 2 board jumps; that's to say 133;111 or 122;101.
But on the other hand, I wouldn't see (in this case) any need to follow "typical" if you feel some decent potential in the upside range of the 101.
I mean, when you weigh it up, do you have more to loose by not taking that upside c/w the extra overlap zone between 133 and 111 ??
Wider tailled 2008 101 probably closes down the gap to 133 just that little bit more than 2007 101, but it's marginal. New fins on 2008 I would rate better in real life than 2007.
Another factor to consider is if you (personally) tend to prefer a "smaller" board in these (7.3) conditions (with potential for a little more performance once powered but a slight extra challenge / more active style to get/keep it there) - or are you a rider that prefers to carry that little "extra" board for most occassions and takes a slightly easier (more passive) approach to planing etc. Both are valid styles, and can influence (personal) choice.
Interesting side point (makes life harder, not easier for you..) is that I know many guys who chose the 111 when they really could have gone with the 101. And vice verca; many guys who probably should have gone 111 but went 101 instead. Good news is that very few consider they went the wrong way, or made the wrong choice. Basically with both 101 and 111 you got massive versatility and range in that "perfect" (not too light, not too crazy) slalom range... you know, just about 7.3m , nicely powered!
Then factor in the cost of the new board and I'll be sure that tilts it one way or other (?) S/miles per hour. Very important factor in these boards too!
Sorry I was not decisive, but hope you got the idea..
Cheers ~ Ian