IPhone 4s - wall charger or pc-usb?

Just wanted to know if there's a difference between charging via usb or wall charger.
I heard that a slow charge (via pc-usb) will be better as it will "tickle" the cells more. Is this true?
What's the proof of this based on electronics regarding Li-ion batteries?

Note:
1. We have 220V sockets in our area
2. My pc has a 650W power supply with a… I guess a power hungry machine since my specs are:
- intel i5 2400 @ 3.10 GHz
- Asus P8P67-M
- GTX 560
- 2 pcs 4Gb RAM DDR3 Kingston (= 8Gb RAM)
- 4 cooler master case fans with 4.2W power consumption each
- 500Gb SATA
- 1 cd drive

You're talking about trickle charging. This is a function of the charger, not the type of socket you plug it into. You can leave a full battery plugged into a charger and it will trickle enough charge to keep the battery charged and alive. It's good to do this from time to time.

Anyway, USB is USB, up to a point. It's all 5 Volts, but the maximum current when connecting for data purposes is just under 1 amp. If it's charging something it can provide up to 5 amps. This is negotiated between the device and whatever it's plugged in to.

The problem is that sometimes, for any number of reasons, the USB ports on a computer can't supply the full 5v for charging. Maybe there's another device on the same bus and the it's drawing a fair amount of current too. Who knows? All I know is that the only way I can ever charge something as quickly using a computer USB as I can plugging into a wall is if I remove every USB device connected to the computer first.

So in theory USB is USB, but in practice some are underpowered. As to whether or not deliberately using an underpowered USB port provides any battery benefits, I doubt it. Your charger, the thing that plugs into the wall, is designed to interact with the phone and deliver the amount of current best suited to charging and maintaining battery life. I doubt a slight underpowering issue will do much either positive or negative to your battery life. There's a threshold below which the phone will tell you that this is not a charging port.

The difference between a slow charge and a fast charge is quite substantial. Charging a LI-ion battery at 5A is considered a relatively slow charge, and the battery is designed to work well this way. A fast charger uses double, triple, or more amperage and while batteries can be charged in 10 or 15 minutes, their lives are considerably shortened. But the choice you have between AC and USB isn't between fast and slow charging. It's between slow and slower.