Not sure about the LifeSaver bottle, having seen one only a couple of times, but it seems pretty heavy and bulky to me and I would probably reject it on those grounds alone for wild camping/backpacking use. The Steripen I have used a lot and find it very easy to purify large amounts of water very quickly. However, it's far from perfect, since it relies on batteries, which can run out, is rather fragile and doesn't remove any foreign material from the water. The last is not usually a problem in the UK mountains, but there are times you might have to rely on muddy or cloudy water.
These days I favour a Drinksafe Systems bottle which is very light and costs less than the Steripen or LifeSaver. Water has to be squeezed through the filter, which is ok with me but some people find difficult.