build it yourself... If you can
I did myself and saved alot of money. Only thing I couldn't do is balancing, but a local shop can do that for you. Not only did I save money then buying a built assembly from DSS or any vendor for that matter, I used even better quality parts like 4340 forged internals and still came out cheaper then any vendor.
my .02 cents
Both companies do a great job from my experience. I have two DSS engines and have been very happy with their work. You can save some money by doing it yourself, but there is a bit more power to be had from quality machining and finish work.
Find a good machine shop, preferably one that works frequently on Fords, if you're going to build it yourself. I'm not sure about Coast, but i'm pretty sure that DSS also offers the rotating assembly and the finished block as a package, and then you can put it together. That would save a few bucks too.
Livernois Motorsport and Kuntz and Co are good choices too.