Depends. Stannous fluoride, the first Fluoride to be put in toothpastes, is an effective desensitizing agent, so if you're cold, hot, sweet and salt sensitive, it's the better of the two to use. Sodium Fluoride is a more effective cavity fighter.
Not much difference. Some studies show that stannous Fluoride is better, but I really don't think there's much difference. They are just different chemical formulas that "holds" the Fluoride ion in a stable form. I think any ada approved Fluoride toothpaste used consistently 2-3 times a day is great no matter what kind of fluoride.
No. An article in the jada (journal of american dental assoc.) concluded that no differences were observed in gingivitis, bleeding, or mean proportions of microbial forms in the stannous fl or sodium fl grps when compared w/ the placebo grp @ 18 mos. Results indicate that 0.4% snf2, or 0.22% naf is no more effective than a placebo in reducing gingivitis. More stain was detected in the snf2 grp though.