migration assistant with WiFi or ethernet cable?

I've just read that one can transfer files between Mac OSs either over WiFi or via ethernet cable and wonder if there are issues I should be aware of prior to attempting it. How long would the transfer take? Which is faster and more reliable? I'll be going between OS 10.6.8 and Mavericks (or Yosemite, depending on how long we wait to make the purchase).

Can I use the same ethernet cable that currently connects my iMac to the wall LAN outlet but use it to interconnect the two ethernet ports on the two machines for this transfer? I seem to remember requiring special ethernet cross-over cables for some type of action in the past (but so long ago that I don't remember why or what I was doing). I'm planning on obtaining a new iMac that will lack a Firewire port and need to think about how to go about this transfer.

I've had experience using migration assistant between two 13" MacBook Pros via Firewire 800 cable that took 2 hrs for the file transfer. The recipient was running OS 10.9.2 and the source OS 10.6.8. Only about 150 GB needed to be transferred.

Thanks to all for advice.