Depends on the task, single threaded apps (iTunes) will run quicker on the 3400 due to the higher clock rate, overall performance will be higher on the 3 core 3500.
The 3500 has a better built in graphics processor and if it is for gaming and you don't add another discrete card then it will play most stuff on low settings on it's own.
Advice would be go for the best you can afford for the tasks you need.
For low cost general computing these APU's are great - if you have the money for a discrete card then get a low end Intel Sandybridge part as the cores are better.
Also remember the FM1 platform is looking to be replaced with FM2 when the new trinity chips arrive later this year, so if you are dead set on AMD it may be worth waiting.