Having built many layouts on hard floors, I very strongly suggest a carped floor for your layout room! Every time you add an accessory or light or more track, you will be getting on your knees and laying on your back (or back side). It is also less fatiguing when standing for a long period of time. As for loosing screws, a strong magnet works great! And if you drop a car or heaven forbid an engine, the damage is usually far greater on a hard floor.
Good luck with your decision and happy railroading,
Don
