I have the 620 and managed to get it to 3.25GHz. The i3, I've heard can get 4GHz but I have also heard the 620 being able to get 3.8GHz.
It kinda depends. The 4 real cores will definitely be an asset in anything that takes advantage of it and the CPU can definitely handle any games quite well. The i3 though is more powerful in each core for sure and has hyperthreading to try to make up for the lack of 2 additional cores.
As you can see the i3 is better in everything except things that are multi-threaded well.
I am surprised you found them both at the same price though; from everything I have seen, the 620 is at $100 and the i3 is at $120.
As a matter of personal preference, I would go with the 620 as I can see the quad keeping it's power longer. I mean, would you rather have a single Athlon FX today or a slower per core but dual core Athlon 64?