I was reluctant to lose my HU, but took the plunge and only have the WS-420sq and have been fine with it. It has the 3.5mm and the RCA inputs so I can swap between 2 phones or whatever. It's been fine. I've had to put in a remote turn on lead to power it on/off, but that was easy.

I put the WS420sq in the spot where the deck was and got a 1/2 din pocket to use up the extra space.