A lifestraw will let you drink water instantly, it lasts for several gallons, and it filters out everything except viruses.
The tablets work great, but you have to wait for them to work before you drink. The tablets also don't affect the water's flavor, but a Life Straw can remove nasty taste from dirty water.
The downsides to a Life Straw is that if the water is really dirty, it can be hard to suck water at first. Also, you might have to kneel in mud or dirt to use your Life Straw in a body of water.
Life Straws are more expensive up front, but they're a better value since they last so much longer than tablets, IMO.
Both are great to have. Use your Life Straw for water you want to drink immediately, and use your tablets to clean water for cooking, bathing, etc.