Hardware

ENC28J60 is powered with 3.3 V and the SPI pins have 3.3 V logic, but are 5 V tolerant.

The 5 V tolerace means it can be directly connected to 5 V SPI and reset pins of host MCU. The 5 V AVR MCUs read everything above 3 V as HIGH. This means there is no need to add level conversion between a 5 V AVR MCU and the ENC28J60.

The ENC8266 is powered by 3.3 V and can require 180 mA as top current draw while sending a packet over Ethernet cable. 3.3 V pins on common 5 V Arduino boards can't deliver this current. Uno and Mega R3 can supply max 150 mA on 3.3 V pin and the small Arduinos like Nano and Mini only around 50 mA. This means, for use with a 5 V Arduino use ENC28J60 modules with 5 V Vcc option and power them from the 5 V pin.