well, see it depends,
i myself have a 32gb 3gs
my buddy has a 16gb 3gs
now he runs FULL blown themes which i think are the dumbest thing ever and my phone is WAY faster then his..
i have safari download manager, music controls, ifile, winterboard, bossprefs, sbsettings and a bunch of others and i dont notice much lag now on his phone there is lag when switching between programs but thats because he has his phone loaded down with all kinds of dumb **** which i feel makes the phone look TERRIBLE. not only that but his battery sucks!