USB or Ethernet

You need to give us more details. I can tell you that using cypress (slave) hooked to a PC and using cypress drivers is not that bad. From start to finish it took me about a month to figure out the C code, the fpga code and talking in one direction, and I had never touched USB before. Now if you want two way communications between FPGA's that's another story and I would think you might want to use NIOS II and ethernet or usb if drivers are available. I wouldn't want to develop a TCP/IP (or even UDP/IP) stack on an FPGA, unless my boss was willing to buy some IP that takes care of the physical layer and presents you something like a buffer or memory mapped interface. Otherwise if you are completely new to it, you are looking at probably a few months of a lot of work, if you're the only one on the project.