Pick your board: http://arduino.cc/en/Products.Compare
Personally, I like the Mega2560 due to maturity.
Before you buy anything, research the required amplifiers needed for EEG. Are you going to buy a headset or build? What electrodes? Are you technically "up" to designing and building the amps? http://www.teuniz.net/12-ch_EEG_amplifier/
Flow ---> electrode ----> instrumentation amps ----> filtering -----> Arduino
I was wondering what is the most valid?
I think you should make that decision. However, if the decision comes down to an Arduino concern, ask your question but be specific.