Looking back, I used to have both the old Sonic 110 and 125, I remember not being able to decide which was best, both worked great with 7.8m and 9m for my 100kg weight. I think you might have the same problem deciding between the 107 and the 117.
This time round, both boards have very different bottom shapes to help decide. The 107 has Flat Vee to Double Concave giving great range of use, and the 117 has flat V for control. The 107 has been bought by many and every one loves the Flat Vee to Double Concave bottom shape, there are lots of reports from owners telling how good it is. The 117 has been out-sold due to the excellent 117w, so not so many owner reports around. I suspect the 117 is still an excellent board though. If you can speak a bit of French, windsurfing33.com has a small amount of info for the 117.
The benefit for the 107 is that for med/heavy weights this board will work in a big range, with tuning it works great in light/med/high winds. Were as the 117 design brief is light weights light wind board hence flat V. The 117w with its extra width is light/med weights light wind board and its Flat Vee to Mono Concave gives big range of use (like the 107) and I suspect that despite its extra 5cm width, it will still be easy to control in rougher waters due tail width/max ratio. A lot of people report that despite is big width, it feels much smaller under the feet.
I would guess that at 73kg, you can get going pretty early on your 59cm board and 6.6m, so I guess you want the 107 or 117 just for light winds. If this is the case then you might as well go with the 117 to maximise light winds, 8.6m for super light days and 7.5m (if you get one) for flukey days.
If light winds are what your looking for, its worth thinking about the 117w as well. This board has no negative reports from owners.
It would be interesting to hear from 117 owners. I'm waiting till 13th Aug to see what the new iSonic look like then might change 3 boards for 2, my iS107, iS137 & F161 for the 2013 117 (or 117w) and US147. But it depends on if there are any big changes to the new iS Speed Specials and if I can stop myself from buying them