both are useless IMHO, they are numbers, but if it was to come to which I would want for my business, I would not choose a .mobi, and I would not pay 500 for a NNN.us either, unless it was a special number like 500.us...
3 number .us's are no big deal..they just get bounced around between various resellers and the price keeps going up until someone get's stuck with them at too high of a $$$ amount...it's like a big pyramid scheme with these types of names, of course if someone pays $200 for one they try to resell it for $500 and so on and so forth...I bought some NNN.us's for like $35 each a few months ago and sold them all for $100 each, tripled my small investment and was done with it..waste your money if you want to! but there just are NOT end users for these names and you can get a NNN domain in MANY other better ext's like even .org for the same money.
cause they are NNN... ie what use would an "end user" have for a set of random numbers? Now if the number was special like 666 555 444 800 etc, then it may be worth more then say 948 784 291 etc.
800.us and anything like 800* ie 800[phonenumber here].us is automaticly reserved by .us registry. The same for 888 866 877.