Another difference is that FPGA (field programmable [logic] gate arrays) can be retooled to do something else (by professionals) and hold their worth in some respect. ASICs, (
Application Specific Integrated Circuit) on the other hand, are hard programmed to do one thing, and do it well. But if the "bubble bursts" on the whole bitcoin mining phenomena, then they are worthless.
If sh*t hits the fan, ASICs are worthless, where as FPGAs are not so worthless.
So, we are speculating that "mining" bitcoins will not go belly-up anyways, so why not ASIC.
Look what I found on google:
http://www.differencebetween.net/technology/difference-between-asic-and-fpga/