PO because it saves a lot of time. It takes a long time to get the Power Items, EV Train the Pokemon, level them up to L100 and get their corresponding items in the Battle Subway. I might plan on going to VGC2012 so I still have a VGC competitive team. But I don't think I'll go back to the tedious process of Wi-Fi Battling unless I ever feel the need to use PokeGen.
I also don't have Wi-Fi on my DS due to a WPA lock so the only way I could battle on Wi-Fi is if I use my Gen IV games on PBR.