far from an expert but here are my thoughts...
Consider the following:
- The type of pack/bag you will be carrying
- The likely methods of obtaining water
Since we are talking urban this could be anything from turning on a tap, patching it from a gutter spout or collecting from puddles. You will be more likely to get nasty chemicals and haz materials from the city. Just think of the ugly slick you see in the water puddling on the parking lots every time it rains.
Honestly with urban you are probably wanna do more chemical or "heatless" filtering due to many factors. How far can the smell of smoke travel? Now think of in an urban setting when someone smells smoke. You may draw unwanted attention to yourself. Also, if you are using a smokeless fuel source like a gas, you now have added a gas canister to your kit = more weight/bulk. You also now have a potential fire hazard that could really wrench your plans should something go wrong.
The tablets work good but as stated they take time to be completely effective. I would definitely recommend getting a Life Straw, or two! It would probably help you out with the dilemma of drinkable water in a SHTF scenario.
Just remember your redundancies!