RE: Replacing a 105L Hyper
Wow. This is one of the best threads I've seen in a while. I'm very humbled to play a role with the distinguished discussants.
Some thoughts on my biases...
I've watched since the early 90s, when I started windsurfing. I've seen a lot of people standing on the shore, waiting for sufficient wind to plane, when it's capping and 'wanting' to go off but not quite getting it. TYPICAL inland lake conditions, even great lakes, though the great lakes tend to be substantially less gusty than smaller inland lakes; the great lakes tend to be more up and down in ~90 second periods. Not as good as ocean sailing, but WAY better than little inland lakes. Lots of side shore to side-on frontside conditions.
In my view, there is NOTHING worse than sitting there knowing you could sail but not having a big enough kit. Getting good at WS'ing is all about time on water, and I think it really pays to have stuff to get out in light winds. EVEN FOR EXPERTS.
I think a lot of Formula advocates have the same view, and have decided to get huge stuff in order to plane in almost any conditions. I personally burned out on big stuff; it's just harder to use on my body than I care to venture (I get blisters on my hands). I'd rather do light wind freestyle or go running / cycling / whatever. So I think a lot of people got underboarded, and the reflex reaction was Formula. That was a good development in that it pushed the design envelope. That said, board designers, and *B in particular, have really learned a lot and offer more traditional volume boards that have range we never dreamed of in 1990.
Without question, I would therefore err on the side of larger board, and experiment extensively with fins, mast base, downhaul and outhaul. With the right fin, heavy railing pressure and the straps / mast way back, sheeted in as hard as you can (sail optimally set), my experience has been that heavy chop is dramatically easier because the board is not in the water very much. It's more like skipping from crest to crest, in a surprisingly smooth ride, but it requires tender foot and ankle work and being massively overpowered while sheeting in as hard as you can. That's my idea of fabulous 6.5 slalom / B&F sailing. You know you've got it right when it doesn't "feel" fast, but nobody can match you and the GPS numbers are impressive. In that mode, I would expect the iS101 to be more versatile, though a tad slower in maximum speed than the 94. I doubt you'll notice the difference, really, without formal racing or a GPS. To do it right, you'll need a fabulous race sail, top end mast and boom, and you can expect to break a lot of masts, booms, and even fins (done all of that, more than once each).
Of course, I'd do the same thing on the 94, but have to live with a 1-2 of knots less wind. Doesn't sound like much, but for Ontario that would translate to as many as 14 days a year, maybe more depending on your local access. I suggest you seek out records from Environment Canada and do your best to quantify what you would be missing.
You've had a lot of good opinions, I think. Nobody has the right answer, it's all what's right for you.