8-channel relay boards have a JD-VCC jumper, so you can choose.
That makes it also easier to power the relay coils separately.
8-relay boards also come in the 12volt coil variety (35mA/relay instead of 75mA/relay).
If you use a 74HC595 breakout board with each 8-relay board, then you can control as many relays as you need with just three Arduino pins. That could make wiring a lot easier (relay boards separate from the Arduino).
Leo..