We need to see what you will be using these computers for and what monitor resolution your using if you want to play games on these.
Both will need graphics upgrades if you want to play new or not too old games. However, the SYX power supply is unknown, so a graphics upgrade might be trouble if the PSU needs upgraded. The IBuyPower, however, has a 480 W PSU, which should support a mid range graphics card, as long ans the power supply is of good quality. For gaming, I would choose this as your best bet. You might want to upgrade the processor as well if you get this, as quad cores are becoming prominent for computers.
As always, know that building your own computer is best for getting the parts you want cheaper than pre-built.
Building your own isn't really cheaper, it is at least as expensive but, you get to decide what you want. If you are content with a pre-built machine, the first one should offer greater bang for your buck. It comes with a quad-core and a decent graphics card. I would take the first one over the second if I had no intention of doing any upgrades simply because it is pretty *decent* from the start. He should be quite set if he uses it for daily tasks and the occasional game.
I buy Dell machines for all my large clients simply because I can not beat their price and, if something goes wrong, it is Dell's problem, not mine. Everything is then warrantied from them and they can offer greater service than I can because they are big business. It is usually better to buy pre-fab computers because they are inexpensive, easy, and have a better warranty than you will get for custom built thus they are a cheaper long-term investment as well. I like custom built rigs as much as the next guy (maybe more seeings as I build corporate servers too :S) but, there comes a time when you need to acknowledge the Big Box computers have their merits. That is why I use them...even when I am totally capable of building many machines in a single day. It is easier to just order them and have them delivered within 3 days, ready to start with the push of the power button.