Gaining strength/speed in the off season

First, welcome to BT.

There is no definitive answer to your question as multiple approaches can be shown to be successful.  A lot of it depends on your abilities and previous training habits and your specific goals for the next season not too mention your race schedule.  If you have an early season A race, you are going to have to hit it hard in the winter to peak well.  If not, then going long/slow may be of more benefit to some.

In other words, there is not a real answer that will help you to decide what will work best for all the variables involved in your own specific training.

what rick said.

however, just for a bit more info, this is what i believe to be the most effective for ME, and many living in areas with a real winter.

the run: should not really be weather effected. lots of easy running through the fall winter.
swim: weather does not change at all. through the fall is more of a back off and then build into maintenence mode, few times a week, short swims, 1000-2000 yards, no more. shorter main sets. just get used to swimming again, real workouts start up after new years.
bike: fall is just getting in some riding as i feel like it, 1 or 2 days a week. noting structured. as winter rolls around, outdoor rides as i can, just used for distance, indoor or mountain bike in the snow, are shorter harder threshold building intervals.

as the weather starts getting nice enough/roads clear enough, i will start lengthening out my riding to acomadate this.