Timber hardness is measured by its Janda rating. Can't remember which is hardest, but if you want a nice floor then mixed hardwood gives a variety of grains without, usually, being too stripped in its finish. Another very good floor, pinkish to red is brush box, more expensive than the mixed species. You also have different grades too, go to borals site, they have pretty good info.
A floating floor is just that, not fixed down, some are good, some are not. The poorer ones tend to swell when wet, even moping can cause problems, you get what you pay for with that stuff.
A tongue & groove floor laid over another sub-floor is the standard system of floor installation now. The tongue is offset so the boards can be secret nailed with a good strip of glue placed on the sub-floor first, then sanded & polished.
Me, I'd go with a tongue & groove, colour & species etc are all personal choices, but remember select grade may often be not too much different in finished quality to standard & better grade flooring.
