The difficulty increase slows as it grows, think of it this way, for the difficulty to double from the point it's at now, all the asics that are in the wild right now will need to double (plus more to cover the gpus and fpgas) that's a lot of asics.
For the difficulty to double from the point it's at now, KNC has to start shipping
Avalon fails after few hundred sets(newer chips are questionable) , BFL does very slow, Kncminer is preparing the sniper but lots of people says not successful if fails to ship the first chip out of their factory on or before 30 September 2013.
If you care money, the get more FPGA.