1. You don't know what they put inside the system. What kind of quality for mobo, PSU and other things. If you build them yourself, you know the components very well. Moreover, you have chosen first class quality components.
2. If you build it yourself, you gain extra knowledge and experience. This knowledge is very valuable. I can earn extra money from building new rigs for my friends, neighbour, or relatives who don't know anything about computer.
Another reason not to get the pre-built is that more than 4 GB of RAM doesn't do anything for gaming. Maybe if you're also doing audio/video/photo editing, it might be useful, but that doesn't sound like it's in the plan.
The difference between a 955 and an i5 isn't significant enough to take a gamble on the other components. At any reasonable resolution, they're going to perform within a couple percent of each other.
Your build looks pretty good, assuming you're not gaming at 1920xNNNN. Don't forget to get an OS with the build.
Yup, homebuilt wins the vote here too for all the reasons Guanyu stated, you seem clued up enough to find decent components, theres only satisfaction to be had from building it yourself, Plus you have a free techline right here in the community