We usually start using smaller studs that are used in car tyres early winter when the snow is not enough or is not covering everything. Then there are frozen puddles and small snow piles that you have to cross, these small studs give you a little security though they dont work on ice (you wont spin around like a jerk). And they are pretty durable even if you ride on clear ground, for a longer periode.
These best-grip nails are by the way interesting, they´ve got different sizes and you can take them out and insert into a new tyre:
http://www.best-grip.com/eng/default.asp
I will probably try them out (if they are cheeper) since Trelleborg prices are going ski high over here!