First, I find all the versions of the Nautilus interesting and attractive (even if I have my preferences, especially concerning the bracelet vs strap issue).
What made me buy my first Nautilus, and go for the 5712, is the thinness.
I tried a 5711 and fell in love with the way it looked on my wrist i.e. elegance with a little casual touch.
This is what mattered most to me at the time.
I love the 5726 too but, as it is a much thicker watch it switches from a nearly dressy casual watch to a sporty one. The 5980 is even more so (and too big for my wrist).
So what helped me decide was the thickness.
If you want something you want to play with a little, a more technical watch, then the 5980 seems more suitable.
If you want something you'll forget on your wrist and can be very well fitting suits, then the 5711 is a good call.
I think you should spend some more time imagining it on your wrist during the day and decide what you want: will the 5711 be less exciting than the 5980? Will the 5980 be too much compared to the 5711?
Anyway, keep us in touch with your decision!
Great project Chris
Cheers, Mark