Both locations are the same distance from my place so I've washed my dog at both. If you want one that's bustling with others washing their dogs, staff is friendlier and more interactive, and afterwords you can easily find a popular and convenient place nearby to grab a drink or snack with your buddy, then head to the one on 7th Avenue
If you have a beast of a dog who is skittish at the dumbest things, thinks she's too good to be near any water, and you don't want anyone to awkwardly bother/pity stare at you while you wrestle for an hour just to put some shampoo on this said beast, then head on to this quieter location. No one will bother you or judge you at this location, not even the employees. I guarantee it.
I also love that they give you a bunch of unlimited free treats to spoil your dog. Beware though, if your dog likes to chase cats, they have 2 orange cats that lounge by the cash register all the time so hold on to that leash tight.
If you live in the Arcadia area, try to get your hands on their coupons that comes in the mail in the Arcadia community booklet. It usually has a coupon for a 5 dollar self wash (savings of 8-10 bucks!) and some percentage off their pet food.