After reading the recent article on UV and gravity filters, I whet surfing on gravity filters and I am intrigued with the Sawyer models. If they live up to their specs and don't clog too easily, they sure add up on paper. I would build a gravity system using a couple Platypus bladders and a Sawyer filter. The Sawyer is just 1.8oz, so carrying a spare filter is not an unreasonable option.
I like the idea of the AquaMira Frontier filter, but it strikes me as having too large a pore size; it makes more sense with a filter/chemical hybrid system.
I use a Steripen with MicroPur tablets as a backup. 99% of the reason I use the Steripen is the time element. Most of my hikes are day hikes and overnights. I start out with a liter or two and I just want a quick way to add another liter if needed along the way. I hike in areas with lots of fast-running streams and lakes and most water is clear and cold. Giardia is my main concern, but being able to cover as many bugs as possible is a plus.
I have confidence in chemical treatment, but time is the factor. With the cold water, I don't think waiting 30 minutes is effective and waiting several hours on a day hike doesn't add up. Hauling a one pound filter doesn't add up either. With the Steripen, I can treat as I go and I always have several MicroPur tablets in my kit for backup. I think overnight treatment with chlorine dioxide products is an excellent way to go.
I do have a Katadyn Hiker Pro filter which I think is great for groups where you need a lot of water quickly and the weight (and the pumping) can be shared.
Boiling is always an option. I don't treat my cooking water if I am low on treated water as I am going to boil it anyway. Topping off a bottle with boiled water makes a nice hot water bottle for cold nights too. If you have a campfire, you can boil as much water as you need for the next day.