There are several reasons an Internet accessed list can be more effective, assuming the system providing the responses to queries has enough bandwidth available to support the requests.
First, changes to the status of a site become available as soon as they are added to the Internet attached system, rather than waiting up to a week after they are detected with Spybot S&D's weekly updates. Second, downloading and searching a list of thousands of web sites locally is inherently inefficient, since it's unilikely anyone will ever access more than a handful of these sites in the lifetime of their PC, let alone a short time period. Third, the filtering can be done with much more granularity, down to a specific page or even file, which also implies the potential for hundreds of thousands to millions of entries in the database which obviously can't be supported locally on each PC.
Also, since the previous design that SmartScreen Filter is based upon itself used a smaller cached list of commonly accessed 'bad' sites, I'd assume it does the same. Since this list was downloaded only when it changed, which may have been weekly to monthly, it only contained a small number of well known bad sites that didn't change very often, which is what PepiMK stated they generally use Immunize for anyway.
So in general, SmartScreen Filter has all of the positives of Immunization and more with virtually none of the negatives. I personally make my security choices based on logical examination of the abilities, not historical determinations of what has worked in the past. This is important since significant changes in the design of either the OS or something like Internet Explorer can have major effects on the ability or even need for some of the older security programs to provide protection. Ignoring these changes simply leaves the protection in an out of date status and may actually reduce the true security overall, exactly the point everyone else believes they're trying to make here.
Bitman