Re gravity filters: take a look at the ULA Amigo Pro. I have one and love it!. It's a little tricky learning how to handle the bag. The first time I tried it in the field I almost fell into the creek and then spilled most of the contents down my legs into my boots! And there's a little trick to getting the bag to stay open while you're filling it. You'll want to practice with it in the bathtub. But after a short familiarization period, it has worked just fine. It's a lot lighter than most other filters, gravity or otherwise.

I know a couple of people who like the Aquamira Frontier Pro filter, which is a coarse filter (3 microns) that filters out only protozoa (giardia and crypto). It won't do anything for bacteria or viruses, but the idea is that you use chlorine dioxide in conjunction with the filter. The filter gets out the protozoa (the critters that the chemicals take up to 4 hours to zap), while the ClO2 takes care of the bacteria and viruses in a mere 20-30 minutes. I certainly wouldn't ever use the filter without the chemicals! Jason Klass has a video on making a gravity filter out of it.