Max zoom is almost unusable for every thing I can think of, at least if
You mean N900 and navigation. Remember, that getting max zoom =! getting all lower zoom levels, unless you choose to download them also.

Nokia Maps vector thing *never* was even close to mapping quality Marble offers. I think You're just mixing technologies here. I also rely on predownloading maps and navigation (You can download navigation instructions for Whole world from Marble, without any problem or too big space usage), I just choose to download tiles I'm really going to use. Like sane zoom (not largest) or tiles ~ fixed kilometers from road, which for Marble got nice option bundled in.

Just give it a try, if You're willing to see difference IMO, no commercial navigation program offer quality like that.