Personally, I would go for the X3430 this is due to ive used the X34xx series for a series of months now and its a pretty amazing CPU.

I've never used an i5 so im not sure , to me an i5 is desktop equipment but then again it depends what your hosting.

3.2TB for the X3430
4TB for the i5.

Both 4GB ram..

Depends what you're going to be hosting with what size disk you need if its a small site maybe look into getting an SSD?

In the question about DoS attacks.. It depends what type of attack if its UDP flooding which will fill the port or Slowloris flood which will overload the webserver..