TSMC is usually a generation behind other fabs when it comes to technology nodes, but they are the biggest fab in the world. Most mobile processors do not need to be leading edge in technology node, and TSMC fulfills a lot of such orders. Most profits are to be made in trailing edge tech. The leading fab in terms of technology node is actually Intel and they are years ahead of rest of the industry but they are not in the foundry business (they fabricate just for themselves). They did however, just take up an order for building FPGA's for another company and so they might be interested in entering the foundry business after all.
The biggest competitors to TSMC are UMC (another Taiwanese company), Globalfoundries (spin off from AMD; merged with Chartered Semiconductor) and Samsung. Globalfoundries is yet to start their US fab (should start later this year but it will take time for them to ramp up). IBM has a fab in US, but they are a relatively low volume fab, as a lot of their production is of their own chips.
All fabs have clients all over the world, and I doubt Apple will choose their supplier based on geographic location. And I doubt Samsung being a competitor had anything to do with them moving to TSMC. Foundry industry is quite competitive and the competition will only heat up as capacity increases. Both Globalfoundries and Intel are increasing capacities with new fabs. The fact that Intel released their 22nm technology to an outsider shows that they are strongly considering entering the foundry business and are either testing the waters or planning to learn from this experiment. In any case, this means that in a few years there might be an oversupply of production capacity on the worldwide arena, which would inevitably lead to lower pricing by fabs.
The moment any fab offers Apple a better deal than TSMC, be it Samsung, UMC or Globalfoundries; Apple will switch suppliers in a heartbeat. Fabs guarantee meeting the design specifications and so a shipped processor from TSMC is no better or worse than one from Samsung or any other fab. Some fabs have lower yields (TSMC had problems with their 40nm) but that is the fab's problem, not Apple's.