The MAX35104EVKIT# is the more "traditional" evaluation kit which includes the USB connection and software to evaluate the MAX35104 rather quickly and easily as long as you provide your own transducers. The MAX35104EVKIT2# comes with two 200kHz transducers and is of an Arduino form factor, meaning it's intended to be "stackable" with an Arduino. The 2# version is smaller and less sophisticated and expects the user to debug the data themselves whereas the traditional evaluation kit does the debugging for you. It should be noted that the performance of the two devices is the same.