most springs are made in the same factory! just painted in diffrent courlors. spring rates tend to vary with each spring made, altough some makes are stiffer than others so the manufactors say. i wouldnt say it wouold be noticeable unless they where really really stiff like coilovers. whitch i highly doubt.
most springs last around 5year if they havent had a hard life, altough if the car is under load or on track, or rough roads like where i live.... i would say 3-4 years. altough depends on a few things like weather condsions etc.
have had 3seats of 35mm springs avo ones are still runing (4year old max) g-max are stilling running (4year max) and kybs snapped (3years) altough the kybs had a hard life...