Personally I prefer the original Mercedes Xenon headlight units. I just installed a pair in my 94 S420. All I had to do is change the two fuses from 7.5amp to 15amp. Everything else just plugs in, very clean. You don't have to worry about cutting original wires or any water leaks.
I think they cost about $1600 for your car. If you don't want to spend that much you can always buy used ones. All you need is a lot of patience, because they are not easy to find. You can check with Potomac or sometimes you can find them on eBay.
If you want the Philips, I have good news for you. There was a guy selling them a few days ago on ebay. He paid $900 some for the kit, but is willing to let it go for $425. It has never been used. Check it out, item #535242962. They never reached his reserve, so I suggest emailing him if you are interested.
As for the difference, it's like night and day. They are awesome and I would never think about buying a new Benz without them.
My brother just bought a new C320 with sport package. He really wanted the Xenon's, but the waiting list was too long. If he wanted to install them now he would have to dish out $2000. He'll stick with the regular lights for now, which by the way don't look bad on the newer cars.