The .us market is struggling and the .mobi is speculation. Your average LLL.mobi will sell for more than your average LLL.us. Without knowing your exact name I would say lean toward the .mobi, as far as speculation it has more potential than the .us in my opinion (don't shoot me that is just an opinion).
However, if the name is left unregistered in both extensions, there could be a reason for that. I would not pick up lots of those types of domain names. Instead, I would take that money and buy a better quality name from the aftermarket. Renewal fees hurt, especially when you have a lot of speculative domain names.