I wouldnt imagine it to be much FASTER . . . I would imagine that the 3500+ would be more flexible and more upgradeful in the future, like, Socket 939 now has motherboards that are both AGP and PCI-Express, while 754 is still stuck with AGP.
The 3500+ is more future-proof . . . if that's worth $150 to you, then I say get the 3500+. If not, 3400+ is the same quality.