The lifesaver is probably the best all round filter on the market. but it's normal cost of around £100 and the size and weight means it takes up a lot of room and is heavy. I know it filters out everything though the military (marines and army) have both tested it even with urine (do not ask) and it has been verified that it works to produce drinkable water (although not recommended even by manufacturer). If you can get one for £38 that would be great.
The aquamira frontier pro is a great general filter, it has a carbon filter and if used with it bamboo filters is a good, lightweight and small option and is a lot better than a survival straw, it fits to many different systems and if you purchase the source water straw (link:
http://www.thegorgeoutdoors.co.uk/product/740-90/Source-Convertube or
http://www.completeoutdoors.co.uk/Source-Convertube ) can be linked into most water bottles from normal pop bottles through to siggs/ nalgene bottles and dromedary or ortlieb water bags. the advantage is cost and weight and size and versatility. I also have a pwp water filter that conects onto an issue water bottle but they are hard to get hold of now a days and that can be used in a nuclear or chemical environment, if you want anything that serious then you have to look to the military market and possibly back to the lifesaver (it is the model that was being given out when I was in afghan and I some how managed to loose mine on tour but it found its way into my kit when I got home ( no it is not for sale
)
With all systems of water filters and purifiers, the source of the water and how badly contaminated must be taken into account, if you have a fast flowing mountain stream near its source you may well be better than a foul peat bog marsh stagnant pool in sheep country. you must take this into account and the likely areas you are going to try and use it, but you can always, if necessary, boil the water ( and set up a distillation process) to be sure as that is the only 100% way of guaranteeing clean water, although most of us get it to a rolling boil for 30 secs-2 minutes to produce safe drinking water