Strats . . . 3 spring vs. 5 spring trems.

[Chistopher]

It's a very subtle change. If your guitar won't stay in tune, heavier strings isn't likely to change that. Likewise if a strat trem can't dive far enough, switching to lighter strings isn't the cure.

 

[El Dunco]

I can’t speak on whether there’s a sound difference but I much prefer the feel of even numbered springs (2 on my 6 string trem guitars and 4 in two pairs on my 7 string FR) A third spring in the middle makes it feel a bit stiffer and the flutter effect from flicking the bar isn’t as pronounced.

 

[beaubrummels]

IME the reason less springs would feel more stable is when the nut and/or bridge saddles are bad. With less springs, the settling time is slower and doesn't bind and give as much in the nut and bridge saddles. With more/stiffer springs, it tries to snap to 0 right away, which binds and gives in the nut and sometimes bridge saddles, throwing it out of tune. But if the nut and saddles are good, then it's just a matter of feel.