Personally I always build my own rigs, you get a much better price/performance ratio, right now we have the i7-5820k that is looking freaking awesome, I'm going to acquire two machines with 16Gb and without GPU for around 800€, it's going to be freaking awesome for Corona.
But if you need a lower price, the best price/performance ratio is a machine with the i7-3770K, you can get it with 16Gb of ram and without GPU for around 550€.
Both machines with closed liquid ref. system, it's cheap nowadays and great for OC the "K" processors.
The thing is that if you get the cinebench punctuation system, two 5820k get 21 points of render power more or less, meanwhile three 3770k get 23 points of render power, so from my POV, if you think in expandability, space and power consumption, the 5820k it's an awesome cpu with a good price.
The branded machines have some advantages over the custom ones, I configure and assemble machines commercially for specific needs, but what I do is configure them and assemble them if it's required by the client, but the warranty comes directly from the seller for every component, not for the entire machine, on the other hand, the branded machines come with some great case design and a good airflow because of that, while I'm limited by the price the client wants, also they give a full machine warranty, not per component, and that could be good if you prefer it.
So in the end it depends on what type of support you want and how important the price/performance ratio is for you.
The only branded machine I have is a 17" macbook pro, and that is because I needed a powerful laptop for mobility and I develop apps with Unity and native android and iOS apps, and it lasts a lot, it's pricey, but components are awesome, the metallic case is great, and the screen is awesome, and the battery is awesome also for a 17" laptop, but as I said, this is because I have some specific need that just a MBP could cover, not just because being a Mac, but because being a powerful and a long lasting machine.
Cheers.
