... I really want the best operation on 40&75mtrs...
What distances do you want to cover on those bands?
The W6TC design isn't bad - it works like an end-loaded dipole on 80m, and a 3/2 wave
loop on 40m. One issue will be getting the resonances to line up in the desired spots
on both bands, though you probably could feed it with 75 ohm coax and use a tuner
in the shack with reasonably low losses.
I'd have to model it to see how it compares to using a 80m doublet (with the ends hanging
down) at the full 75' height. I suspect that the doublet will have a slight advantage on
80m, while on 40m the doublet would have a narrower azimuth pattern with a greater average
height, while the loop would have a wider pattern with a bit of broadside gain (though not a lot
with the sides spaced only 1/4 wavelength.) The doublet would need to be fed with open wire
line, while the loop could have coax from the end of the open wire line to the tuner.
But overall the differences between them probably aren't too much, and there aren't any
glaring flaws that I can see offhand in the W6TC loop design. If it fits your needs better
than some other options, then go ahead and give it a try.
I encourage hams to experiment with different antennas to find what works for their own
specific situations. For such temporary antennas I usually use stranded, insulated hookup wire,
or whatever you can find locally that is relatively cheap. Once you find a design you like then
you can consider building a more permanent version, as the plastic insulation on most hookup wire
degrades over a couple years in the sun and small wires are more prone to breaking. But it is
light, convenient to work with, and easy to reuse in later antennas.