mbed vs Teensy 3 vs other options
When faced with the same problem, my choice was the "Electric Imp" 


It has a similar processor to the other boards you mention
Wifi Internet connectivity is core to the way it works 
It is low-cost 
It is low power 
It uses a standard form factor (it is mechanically identical to an SD card)  
It is designed for large-scale commercial implementation so may lead to some work 
It has some really useful ports including gpio and i2c (so I can connect to ready made circuits)


http://electricimp.com/