I think either one would work, chose the one that you like doing better. plus you can always switch it up, its better not to do the same thing everyday so maybe a few days walk briskly and a few days do intervals
Both fast walking and running are good aerobic exercises. They both improve overall health and fitness. Choosing which one to do depends on the situation. Personally, I'd go for walking then progress to running once the body is used to the movement.
1) interval training rocks. it does, for tons of reasons, do a google search. basically you can burn a ton of calories by doing short bursts of max intensity.
2) walking at even a low-ish intensity for 60mins a day can lower your "set point". studies seem to show that people who walk, even at a fairly low intensity for 60mins a day were able to lower their set point.
Depending on your needs I would apply both. Are you stuck at a plateau? Make sure to do more longer walks maybe? Just trying to burn the most calories, definitely interval training.