I love gravity filters. Only way to go IMO. I set it up when I first reach camp and by the time tents are up, I've got group water for the evening on-tap.
The katadyn base camp is a good system. The bag is burly and easily cleaned. The problem with the base camp filter is that it relies on the Hiker cartridge. Work with me here: the problem is that the filter is driven by hydraulic head pressure right? Well, that filter only works when fully submereged. So, you have to completely fill the bag and it only filters till it reaches the top of the filter, leaving a gallon or two of water just sitting in the bag.
The platypus system is nice, it's fast flowing. But the bags are a real hassle to clean or seal. The ziplock style closure is a real bear to operate in the cold. And it's much lower capacity.
Lately, what I've been doing is using the bag from the base camp filter, but not it's cartridge. Instead, I use a sawyer in line filter. (see Sawyer 3-Way Inline Water Filter - Free Shipping at REI.com
) This filter is much faster than the hiker filter. It's much longer lived (if you don't let it freeze) as it easily serviced via backflushing, even in the backcountry. It's light, fast, bomber. I use an MSR dromedary lid/hose kit to mate the base camp bag with the sawyer filter. Some cheese cloth over the threads acts as a prefilter.