Honestly, though, if your 7.3 is in decent shape... swap the turbo to the 7.3.
The 6.9 never came from the factory with a turbo; what you found is a motor someone swapped a turbo onto already.
IF the 6.9 already has headstuds, or you are willing to install headstuds in it before installing it... go for it. If not, use the 7.3.
The N/A 7.3s can handle about 20 psi of boost before blowing a gasket. This is more than a normal turbo kit can put out, period.
6.9s, well, 6 is ok and more than that is pushing it... without headstuds. With headstuds, a 6.9 can handle 20-ish psi.
With headstuds on a 7.3, the headgasket is not the limit anymore. A 7.3 with headstuds will handle 50 psi of boost without blowing the gasket, but the N/A-spec connecting rods can't handle 50 psi of boost(and the fuel to match - note that at this point you'd be pushing well over 400rwhp/500+ crank HP)
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On question two - how much power will a turbo add, the answer is: how much are you going for? Also, what aftermarket turbo(note: Even if you find a "ford" turbo on there, it was off of a 93-94 7.3 and was made by ATS).
Stock, an idi will put somewhere between 85 and 125 to the wheels(this is after drivetrain losses, ac, ps pump losses, etc which is why it's much lower than the rated 180 hp).
Using a stock IP cranked up, and a turbo pushing 12+ psi of boost, you can get oh... 200-220 rwhp. Somethin like 95 hp at the wheels, near doubling what's available.
Beyond that, you are looking at custom pumps(R&D makes really good high power pumps) and bigger turbochargers to put on the kit. Obviously, at that point, headstuds are required.
I've personally pushed a stock 7.3 motor to 250rwhp for almost a year now, using a rd2-110 IP and "factory turbo"(ford-spec ATS) kit. This is very interesting to drive because the torque curve is totally turbo-limited. So it makes peak torque at like 2600rpm and peak HP at 2800. It starts pulling hard around 2K rpm when the turbo starts making boost and doesn't stop until it hits the governor and defuels around 3K.
