I've done it both ways. The garden hose is effective if done right but takes forever. The only way that the coolant flush you get at the shop is really any better is that its done with the engine hot and the system pressurized so you get more crap out of the system, brake more scaling off the walls, etc.
You aren't getting any better water quality since the shop is gonna mix the coolant up with the same hose water you will. Most shops will try to sell you a BG coolant flush kit. I don't work for BG, but i do recommend getting one either way you go. You get a cleaner that helps to break stuff off the wall, you get a conditioner that helps keep your hoses and gaskets in good shape and helps heat transfer, and you get a sealer that helps keep it from leaking.
I take mine in and use the flusher when I need to change the coolant, but I also get free labor.
Hope this helps you make your decision.
