ok, lets say that the frame size and geometry is right for the person being set up. they race at IM level, and want to know what suits them best. 178cm tall, 71kg, and average flexibility. (i beleive that knowing their flexibilty is paramount)
what would you guys suggest, i am interested to know. and to rule it out, lets just say that spares for both sizes, including larger chain rings, are both as easy to get as the other.
I think we have solved and agreed this one already
650C up to about 5’10" maybe a bit more if you’re flexible.
700C from about 5’6" and up.
The overlap takes in to consideration such things as existing gear, flexibility, preference (650C bike makesme look bigger! and therefore more intimidating, at least until i get off the bike 
)
IM distance really just plays with the flexibility issue. For example, I can get into a great position, but after a while the back gives out. Therefore I’d suggest that for IM races, the 650C size might drop to 5’8" (or you could just raise your bars) and the 700C would stay the same.
Now back to our regularly scheduled programming.
TriDork
Tridork is not such a dork. He/she is right. My feeling is that wheel size is another sizing tool and that wheel size is tied to wheel base and overall distance above the ground as well as weight distribution.
According to many “rules of thumb” I am right on the border of 650c or 700c which implies that I cold ride either. I have tried both extensively, on over 30 bikes I have owned specifically for triathlon. I have discovered I am faster, run better and can handle the bike better with 650c wheels. My road bike(s) is 700c, but I stick to 650c on my tri bike.
Again, I think wheel size is a fit tool. It is linked to torso length, reach, etc. Those are some of the determining factors.
I hope that helps. Happy Holidays.
He/she?
Hey, it’s just that the water was cold. Alright?! 
For future reference, TriDork is all man. A bitter and twisted middle aged man. I just RUN like a girl, a slow girl, with a bad leg, and a fat ass, and…
TriDork 
thanks MEN, (tridork, you like that huh?) hehehe, ok, i’m glad none of you brought up rolling resistance, because i beleive there is less, just like in the rolling resistance test in the tech article. i go fast downhill on 650c, but i suck up hill, which was never the case on my old 700c machine.
That is interesting since I found it to be the opposite. Climbing is what I do most poorly as a cyclist but I climb much better on my 650 wheel bike than I did on 700s. I found that when rolling downhill that I have to start pedalling sooner with the 650s since they lose momentum more quickly (and get up to speed more quickly which is why they climb better).
I am 6ft tall and have been riding a 56cm frame with 650 wheels (new frame with 78 degree seat tube, old one was a Cannondale R700 with a 75 but seatpost turned around) and use them for all distances and all of my training. As for flexibility, I had back problems when I was younger and still ride it pretty comfortably for ironman races with a pretty substantial drop from saddle height down to the bars. I bought into 650 wheels when they were the size to get back around 1994 and figure that I’m on the edge of the height range for which they work (I can remember seeing old P2s around size 60 or 62 with the little wheels and they looked kind of funny but a 56 doesn’t look too much like a clown bike) and since I don’t want to replace my Hed wheels 650 is the way to go.